I tried it out in the recent PvP season and it was clearly the weakest monster in my team, due to its reliance on sleep for the whole moveset to work. I ran into so many counters to sleep where it fell completely flat. My Soulstealer was actually bugged at the start of the season (VKC had to fix it) so I temporarily played Metatherion in that slot and found that he actually performed the role better.
In SotF I’ve been trying it out with Onyxia in FL OoO and it’s done very well. However, I’m doubtful that using it in the FL will do well in PvP. I mean, OoO + Soulstealer is going all-in on sleep and you’re very passive it it doesn’t work (especially if using Onyxia) so I’m unconvinced it will be strong.
I’m very intrigued by you saying it’s working well for you! Can you elaborate some about how it plays?