After playing Battlegrounds a bit on the PTS, I've become a little skeptical of the long-term viability of offensive Magicka Necromancers in that environment, and that primarily comes down to the class' mechanics. (Note that I say "long-term" because I'm assuming that various numbers will be getting tweaked and balanced as time goes on; ie, I don't expect Blastbones to be hitting for 6-9k damage in BGs by the time Elsweyr goes live.) Stamina Necromancers may very well have big issues as well, but they at least have access to the overall Stamina toolkit, which is quite strong.
I'll start with Blastbones:
While the damage is currently too high, and its ability to "body block" is a problem, the spell is really hamstrung by poor pathfinding, lack of speed, and potential bugs (on more than one occasion I've had an apparently not-stunned enemy Blastbones stand in my face for >= 1 second before finally exploding). The skeleton's poor mobility can be a real issue in some terrain, especially if you're actually trying to use the ability from range. The most obvious anecdote I can think of was from Foyada Quarry earlier: I was standing on the rocky area below one team's base, and an enemy Necromancer summoned a Blastbones from the platform below his spawn. It navigated the terrain at such a leisurely pace that it never got more than maybe 2/3 of the way to me before the timer ran out, even though I was standing flat footed and staring at it, making no effort to escape or CC.
Counterplay is great, but I think it's a very real problem that simple terrain and general movement - even that which isn't a specific attempt to avoid the Blastbones - can defeat a skill that Necromancers will absolutely need in order to kill anyone. Furthermore, the Necromancer won't even be able to abandon their first target and fully switch to another until the Blastbones' duration expires. There have been a number of times that some Stam build goes sprinting away from me, leaving my Blastbones greyed out for what feels like an eternity.
Next up - the corpse mechanic has several issues:
1) The duration of corpses generated by Necromancer abilities is too short. If the timing of your "pet" or armor buff running out doesn't just so happen to line up with you needing to use up a corpse for an ability, you're...boned
2) Corpse location restrictions in the highly mobile nature of many Battleground fights. If you strictly play Domination games, or defense in Capture the Relic, this may not be a huge deal, but otherwise it's definitely a problem. That's especially true for abilities like Renewing Undeath, where one needs to actually have a corpse inside the spell's area for the cleanse + HOT portion of the ability to function; I find it quite difficult to make good use of the ability in non-dueling PvP, especially since many of those debuffs will just be instantly reapplied (even via effortless procs).
These two issues combined make the corpse mechanic quite difficult to actually use at all, much less effectively, in Battlegrounds (and probably many Cyrodiil fights as well).
Then there's the problem with Magicka Necromancers having extremely limited offensive options, especially if the problem(s) with Blastbones don't get corrected. All you really have, insofar as controlled/targetable abilities go, are the Blastbones and Ricochet Skull.
1) The damage on Mystic Siphon is incredibly low, and will be very hard to land even if that goes up + the above corpse mechanic issues get taken care of. I saw ticks of ~200 in Battlegrounds, and that's when I was actually able to get it to hit someone. Perhaps a damage increase + area pulse around the caster would make it a bit more usable?
2) Boneyard's small area of effect makes it not really worth using. I've played around a bit with Avid Boneyard and using my own synergy for some added burst, but it doesn't really work very well. Not only is it pretty hard to actually make connect, but the damage seems fairly lackluster even when it does land. It could be used to inflate your scoreboard numbers in some matches, but it's not truly effective for Battlegrounds PvP.
3) Skeletal Arcanist's damage currently seems to top out around 1.2k or so for me, and is oftentimes closer to 900'ish. I might could get it a little higher with a glass cannon set up, but not enough for it to be worthwhile - especially since Magicka Necromancer has nothing comparable to Streak, and will thus get dogpiled and exploded. And since the Arcanist always seems to target the closest enemy, and can be line-of-sighted and whatnot, it's actually often worse than simply having a casted DOT that does the same damage (and who would really want a DOT that ticked for around 1k every 2 seconds?)
4) Hungry Scythe certainly doesn't look like a viable Ricochet Skull alternative, since the damage is far lower and requires melee range (and Magicka Necromancers don't get the tools necessary for that playstyle). The health-scaled HOT attached to the Magicka morph is far too weak to be worthwhile, as well.
Stamina Necromancer builds don't really seem to suffer these same limitations. They get access to the same two worthwhile abilities that Magicka setups do, as well as the full Stamina toolkit. There is no Carve, Rending Slashes, proc bleeds, etc...for Magicka builds to stack with Blastbones and their spammable; just a Wall of Elements that everyone will move out of (especially after the nerf to Wall of Frost), and perhaps Destructive Clench/Reach (which, when not used as a Spammable with a Master's Staff, isn't great damage).
Finally: I assume it's a bug, rather than design issue, but many times I'll have corpses generated by other Necromancers (and obviously there's a ton of them out there right now) appear as though they're usable by me, even though they aren't. They'll look the same as mine, and cause certain abilities to light up when targeted, yet not be usable. (I was actually able to use other people's pet corpses after they finished up some duels outside of Wayrest, though.)
I was really hoping that Magicka Necromancer would be a viable replacement for my Magicka Warden, due to the PvP frustrations I have with that class, but so far it feels like a bit of a letdown.