I actually really like him, 4 guaranteed kills if set up correctly and then u can sweep through, okay he’s vulnerable to sleepkiller but he is also pretty tanky, i did the last few ultimate Challenges with him and only one other legendary the nightrider but soulstealer always took out 7-9 mon, i dont think he is as bad as the People think he is, much better then the assisted and the dream hunt legendarys, just needs a bit of setup
If by dreamhunt legendaries you mean Shadowstalker and Stormloch, I’d actually choose them over Soulstealer, since they have hold ground and bloodthirst plus they aren’t super hard to setup either. I’m not saying Soulstealer is really bad or anything, that’s just my preference.
It’s not that he’s bad per se, it’s just that he requires more effort and set up to achieve the same thing that other legends do a lot easier. A full power assisted wave will wreck most teams, and that’s repeatable with a 200s cooldown, less if you have give turners. Soulstealer can kill 4 monsters but it takes 400s and is not repeatable, plus sleep all is a little too inaccurate to just spam repeatedly without being timestriked
I have paired him up with 2 sleep bombers and a super epic protect/absorber and he kills more mons then my rexo lol, the dreamhunt mons are 2 squishy and their bloodthirst needs 2 kills to trigger, and u should never use sleep all with soulstealer, and Tell me how its easier to kill 4 mons with flarevern or cybereon, i understand that especially the stunning entrance mons are better because they are good in literally everytime but kill 4 mons and ignoring their hold grounds with a triggered 4 kills bloodthirst seems damn good to me
Each to his own 
With sleep bombers and protectors the dreamhunt mons can get the 2 kills to activate bloodthirst if not more. As for them being squishy, it’s true they have less defense than Soulstealer, but hold ground can make up for that, plus they actually have a decent amount of hp. Also, keep in mind that Soulstealer has a good less 1000 something points in his attack stats than the dreamhunt mons. Nothing against Soulstealer xD I just like the dreamhunt mons
I love this minster, I combined them with one sleep bomber, one super epic protect and Onikage (flash stunner). First with the Soulstealer I do the death sentence, protect them with the super epic protecter, do stealthy companion with the sleep bomber for protect Soulstealer, stunner flash for gain time, and after the stunner flash, I use the sleep bomber. After that, Soulstealer killed 4 monsters and activate bloodthirst, full of life. Never use ‘Sleep all’ with this monster.
I like soulstealer if it is in my rival’s team.
Okay, Soulstealer is pretty good if you build a team around him. But when you dont have the right support monsters he is indeed not worth a rice ball 
If you really want a death sentencer just use one of the super epics. There are better sleep legendaries out there like Shadowhunter and Stormloch. Not that they are the strongest legendaries, they are just good sleep legendaries…