I did some plays against Gloomweaver Saturday and Sunday while watching football. To me, his biggest issue is that he doesn't put any pressure on you hardly at all. He reminds me of normal Baron Blade in that he has a few things that are dangerous, but he doesn't stop your setup at all. You have almost as long as you want to setup. He has very few "destroy all ongoings/equipments" (maybe none at all other than some choices heroes have at start of villain turn), so once heroes setup, they are good.
I think his biggest combo against me in all of my plays is in Megapolis with one of the +1 damage environment cards out. On villain turn, he Vast Following (I think that's the name), which grabbed the relic that powers up zombies and the the Chosen Disciple and put them back into play, which then grabs a zombie from trash and puts it into play, and then played a disciple that did H-2 of fire and H-2 of cold to all non-heroes). Because of the +1 damage, my heroes actually took some damage. But then on my turns, my heroes easily destroyed the cultists and zombies and everything else was smooth sailiing.
Villains usually win in 2 ways. Heavy pressure and wins via damage, or prevents heroes from setting up, and then overwhelms heroes due to that fact. Iron Legacy and Chairman usually wins via heavy pressure, where heroes can't keep up. Citizen Dawn tends to win by trading back and forth with heroes, and then exploding all ongoings and equipment. Omnitron (both versions) tend to do this as well. A few villains win because they outlast you. When I play against Akash'buta, I've lost just because she has SO MUCH HP that at some point, everything turns against you. Like environments comboing with her stuff.
I feel Gloomweaver tends to try and outlast the heroes. He doesn't really disrupt that often. He forces some discard, but lots of it is optional (discard or take damage). He doesn't have an ongoing where if heroes discard, he damages the heroes (that would be really sweet for him). He has pins that deal damage, cultists that deal damage, and relics that disrupts, but none that swings crushes the heroes in one play. The only card I really fear is Vast Follwing, because it brings back relics and cultists for a potential haymaker of a villan turn.
I've never had him flip on me before, but it still doesn't seem that terrible for heroes. I did give up against him once when I destroyed 2 relics, and he drew out his deck while the 3rd relic was in play and reshuffled. It took me a turn to realize I can't beat him via relics since the other 2 got shuffled back in.
Changes I would make with Gloomweave is either more pressure, or more disruption. I think more disruption works for GW's theme better, more specifically, discarding (your hand is your mind and your options). GW's villain card should have something like "when a hero discards a card, GW deals H-2 irreducible to that hero". Since I haven't seen him flip before, I can't say how powerful he is when he does flip. I haven't tried advanced yet, but I will soon. (I find GW very boring to play against, so I start auto-piloting when I play against him).