So I've been playing around with Sevagoth a for a few days now, dumping loads of forma into the guy. He's fun, but my main critique is that he is insanely too reliant on Gloom to survive. Now, normally this isn't a problem.....except when you run into a faction that prides themselves in Nullifying abilities.
From this point on, I'm making my judgement based on Steel Path gameplay. So, if all you do is deal with Grineer all day, even on Steel Path, you won't find an issue whatsoever, they are a cakewalk once you get a good build, since they have no way to combat gloom. Infested you just need to watch out for disruptor hookshots, mainly, because they will drain your energy to zero at that point.
Corpus on especially on the new tileset (which is now any time you are doing a "Corpus Ship" mission) are a nightmare. They deal a lot of damage because, for whatever reason, every crewman that spawns is using a Supra...you know, the weapon of a Corpus Tech? High fire rate, lots of damage, so you NEED Gloom. Problem is when nullifiers come in. On their own, no problem. But, on the new tilesets, nullifers either sport rapid fire weapons (which also do some considerable hurting), or go in with melee, with the latter having a better chance of covering a group of twelve crewman (Steel Path higher spawn rate), all sporting Supras, to ignore your gloom and gun you down. What also does not help is the range of Gloom, which makes him an open target in much larger maps where enemies can shoot well out of Gloom's range.....like in the new ship tilesets. Essentially, don't take Sevagoth on the new Corpus ships in steel path, unless you are really on your toes.
Now, I get he has a passive to revive himself....but you shouldn't have to be doing it every thirty seconds (exaggeration, but you get the idea). It gets annoying to be forced into a gimped version of a Shadow, who also can get gunned down easily if you aren't moving around like a chicken with their head cut off. Even when using the actual shadow and being tanky with natural stats and a first ability that's solid crowd control, it's not going to do anything to a group of Surpa wielding corpus underneath a bubble. Adaptation and natural armor is not going to last long if you aren't careful.
But, of course, you can avoid all this by using a niche mod for a Miter. Go for it.
The above example just emphasizes a point that many posts have mentioned. Sevagoth really needs something aside from Gloom to keep him alive, unless you intend to just die a lot and use a handicapped Shadow. He is too reliant on one ability to really contend in survivability, and with the game introducing ability killing mechanics more and more often, he loses his effectiveness very quick. Some people have already proposed ideas as to how to do it, so I'll abstain from suggestions of my own.
Other than that, he is very solid.