EDIT 2: For discussion on impacts from DB Update, click here.EDIT: For discussion about the change to SA in the 2.3.3 patch notes, start here. Discussion about Leeching Strikes alternatives start here.
Siphoning Attacks has always been the bread and butter for NB tanks, who themselves have been the persistent tanking underdog. This one active ability enabled the excellent resource management that has been the defining feature of NB tanks. Without SA, NBs have no other class active abilities (and only one passive, which requires killing an enemy with an Assassination ability to gain back a small amount of Magicka) that return Stamina or Magicka.
In addition to its own utility, Siphoning Attacks (alongside other class abilities like Mirage) opened up more gear choices to NB tanks beyond those typically used for other classes, such as not having to rely on Engine Guardian for resource management (or Hist Bark for dodge). SA's reliable way to sustain resources also allowed NB tanks to contribute a strong support role as effective off-healers and/or off-DPS (depending on Magicka vs Stamina spec) to make up for their relative lack of survivability or group utility that other classes bring (e.g. CC, damage shields, direct heals, mitigation bonuses, etc).
Over a year ago we learned that Siphoning Attacks already has a hidden internal cooldown of ~1 to 1.5 seconds (hard to measure for sure). Siphoning Attacks has received significant negative changes in the last balance update (the Imperial City update), which consisted of (1) removing the toggle in favor of a short buff without a damage penalty; (2) adding a magicka cost to this buff where previously it had no resource cost; and most significantly (3) replacing the % of resources returned on the 10% proc on damage abilities with a flat value (1944 Magicka and Stamina). These changes to Siphoning Attacks were on top of two other noteworthy changes that affect the effectiveness of SA. The first is the game-wide change to blocking, which now stops players from regenerating Stamina while holding down block (and it takes ~1.5 seconds after dropping block to start regenerating stamina). The second is the increase of time between damage ticks from .5 seconds to 1 second on many (all?) ground-target AoE DoT abilities, such as Twisting/Refreshing Path (a staple for NB tanks to help them take advantage of the Shadow line passive bonuses). This meant that the amount of times SA could proc on GTAoEDoT (ground-target area of effect damage over time, that's a mouthful) abilities was cut in half.
So, with all of that information in mind, here is the description of the change from the patch notes:
Siphoning Attacks (Siphoning Strikes morph): Fixed an issue where this morph could proc its resource return on every tick from a damage over time effect.
We know that before this change SA already only procc'd off
certain ground-target AoE DoTs after the initial hit. Twisting Path worked, but Refreshing Path didn't, for example. As a single-target DoT, Cripple already has never proc'd it. Caltrops was somewhat different in terms of what it counted as (used to proc basic attack tick, then that was removed with IC update so it only had the % chance to proc). While DoTs were inconsistent with how they proc'd SA, they all nonetheless had a chance to proc it on their first hit (ie akin to a single direct damage ability). Given this, and like
@hrothbern pointed out, I would imagine this change to SA is part of their ongoing effort in making combat systems more consistent.
In terms of balance, the main issue with SA proccing off of GTAoEDoTs was coupling it with Caltrops. Because of Caltrops' size and duration, it was possible to get a lot of procs off of it with multiple enemies. It was a really effective combo, if perhaps OP in some situations -- though much less so since both SA's and Caltrops's tweaks in the IC update. It's also important to state that Caltrops has a very high up front cost, which offers somewhat of a balance for the resource gain SA could provide off of it.
However, allowing SA to proc off certain GTAoEDoTs served a very important function: it allowed you more chances to return resources, especially given that SA has a somewhat high cost for what it is and since it restores a flat amount. Most importantly for NB tanks, it allowed you to do this while blocking. Having a way to help sustain resources while blocking and casting your main abilities was key for endgame content where you can't afford to drop block very often (or at all) because boss attacks are not communicated well (or there are too many visual effects going to catch the animation/telegraph), mechanics are buggy, there are attacks from adds at same time as boss, there's a risk of hard CC (eg knockdown) from adds and/or boss, etc. This is what made NBs able to uniquely and successfully fill the tanking role. Without it, NB tanks don't bring anything to the table that can't be brought by other classes; not only that, but the new inability to effectively manage resources poses a risk to the group.
I tested out the changes to SA and found that my NB tank's abilities to manage resources were severely diminished. Of course, SA still works nicely with light attack weaving, and AoE direct damage abilities like Sap Essence are still a good way to get multiple proc chances. Those things haven't changed, but those things also possess the same problems as they had before: you can't always afford to drop block to use basic attacks, and you can't always count on having multiple enemies close by to increase your chance of getting resources back. You have to get the ability to proc twice (at 10% chance per proc) within 15 seconds to cover the initial cost! Blockcasting a cheap ability like Swallow Soul or Entropy is a very risky way to try and regain resources as you dig yourself further into the hole of resource cost while the single-target proc can't return enough to even cost its own cost.
This change, on top of all the others made since last summer, puts Siphoning Attacks, and thus NB tanks by extension, in a very bad spot. I have several possible solutions, some of which depend on how much ZOS wants to keep it so that GTAoEDoTs don't proc SA (though DoTs can proc other things, such as the Valkyn Skoria 2pc bonus). Note that I'm not advocating for all of these to be implemented, just that they could be considered individually or possibly in some combination to help fix the place NB tanks are now in.
- Add ground-target abilities to list of things that can proc SA. This could even be limited further to class ground-target abilities. Since ground-target AoE DoTs can't be blocked, it makes sense to me that abilities like Twisting/Refreshing Path should proc it. The size of our class AoE DoTs is also much smaller than Caltrops, and thus poses less risk of mass proccing. In addition, NB tanks make use of these AoE DoTs already for the damage, utility, and Shadow line passives, so allowing SA to proc off them would also support common NB tanking abilities. Non-ground target AoE DoTs, such as Cripple, Lotus Fan, or bleeds should not proc (the same as it has been).
- Since SA already has an internal cooldown, add a cap to the number of targets that can simultaneously proc SA. This could be tied to the AoE cap, for example, so that only 6 targets have a chance to proc SA.
- Increase either the value of SA's stat return or its proc chance. If there are going to be so many limitations on how you can get SA to proc, then the payoff needs to be much greater. Consider returning SA to its original form where it restored a % of total resources.
- Reduce or eliminate the Magicka cost of SA, OR increase SA's duration to provide more chances to at least recover up front cost. I would argue that the duration should be increased anyways, as NB tanking since the IC update has become a 10-15 second buff management mini-game. Juggling a half-dozen buff timers is a chore.
- Return SA to a tank-specific ability by re-introducing a damage penalty, or otherwise reconfiguring the ability and its morphs. /u/Gottbeard from the ESO subreddit suggested ZOS could follow the precedent they are setting of taking the popular morph and pushing it to the base skill then introducing new morphs. This would make Siphoning Attacks the base skill, and free up the morphs to specialize for tanking vs DPS roles.