About me:
Let me preface this discussion by stating that I have 1 character of every class in both mag and stam and that, outside of magblade, I regularly play each class at least once every 30 days in order to get T3 rewards. As a purely solo player, this actually amounts to a lot of time since I do not collect a lot of defensive or offensive ticks. It would be safe to say that 80k of the 100k AP minimum that I make is purely from killing other players. So while 100k AP is a relatively small amount, the manner in which it is earned is relatively slow, and not just because of Cyrodil lag. Especially with the current barren/complete zergfest state of PvP [PS4 NA], I spend quite a lot of time fighting in various types of situations on each class. /credentials
Of the 9 sub-classes that I play, 4 use Blood Spawn; magDK, stamDK, stamSorc and stamDen. My stamPlar and stamNB would also be in Blood Spawn if not for the simple fact that I don't want all my characters to run the same set. In fact, I had made it a point to not have a single set (including Master Bow, Willpower etc.) used in more than 1 build and still be competitive within the solo scene, with each class performing at similar levels, even after accounting my varied skill/experience amongst classes. Despite my reluctance, I eventually had to concede to using Blood Spawn on several characters. This is the only set I have had to compromise on.
Now let me be VERY clear and state that this preference for Blood Spawn is 100% NOT because the set is over performing. Rather, the prevalence of Blood Spawn for myself and I [rightly] presume other solo players is that there is no valid alternative that falls into a similar niche. What makes Blood Spawn great is not the fact that it provides ultimate nor the fact that it provides strong mitigation with solid uptime. It is the combination of the two, but not for the specific benefits themselves, but rather because the set serves as a hybrid option that provides both offensive and defensive benefits, something that solo builds are built around, especially after all the direct/indirect nerfs to the play style. The values of the following sets are simply not up to par due to mechanics or numbers.
Ice Heart
Now I would LOVE to run this set on my magPlar for solo open world, but the properties of PvP shields coupled with the built in cooldown and the mechanics of the set make it useless for solo PvP. Buffing this set to enable it to actually provide benefit after a single GCD would bump diversity. Giving the 1 pc bonus both weapon and spell crit would also help it compete with Blood Spawn. With the latest shield changes, the set has more value, but the cold damage should persist after the shield goes down (does not need to be full 6 seconds, maybe 1 second after shield breaks) or have a high chance to inflict Chill.
Maw of the Infernal
A potentially great set if it wasn't for the fact that the proc rate was so abysmal. The Daedroth damage is fine (I believe targeting was fixed) and the LoS would be very useful if the thing proc'd more than once a fight. I have near perfect weaving and the set still does not proc often enough. The uptime simply is not where it needs to be. Also, if we could get this to work with natures embrace <333333
Scourge Harvester
Honestly, this isn't a bad set. I think it just has such a bad stigma. I haven't tested this set in several patches. Maaaaaybe bump the damage/healing slightly as I believe it [healing] is reduced by Battle Spirit twice. Need to test.
Shadowrend
Shadowrend is a solid set. A little too niche imo but solid none the less.
Thurvokuhn
A very interesting set. Great for group play. Maybe have the effect linger for a duration inversely-based on the size of the pool. The poison damage is useless. A snare would be more beneficial. Not much experience using the set, but playing against it is very easy and seems like the set under-performs. If the set is performing well at a Group v Group level than please disregard.
Obviously, the alternative is to build slightly more tanky/glass with the rest of the gear and compensate with the monster helm. This is a valid option for mag classes. One could even slot Za'an if they're ok with a set outputting more damage than a single player can do within a small time frame with a single proc, even as a 50k hp tank with 2 spell damage and 2 defensive sets. However, stam proc sets are virtually useless in PvP. The counter-play, while necessary, is too easy to accomplish and severely diminishes the value of these sets.
Especially from a stam perspective, many of these sets need to be brought up in terms of performance.
Introducing stam oriented monster sets that follow the same approach as Blood Spawn would also go a long way.