Not a very good card; most decks would be better off with just an Avengers Mansion. As a long term value play, the Suit and the Mansion both provide you with +1 card per turn on a 4+card upgrade/support. For an extra encounter card every turn, you get +1 to basic powers and +10 hit points. Encounter cards vary greatly in their effect, but certainly almost every encounter card is more impactful than something you can clean up with just a single point of ATK or THW. So if you're running this, hopefully you can get 3+ uses of your stats per turn, so it's at least in the ballpark of the damage an encounter card is causing. We're talking multiple readies (Quicksilver, Ghost-Spider), or a deck playing events that total up your stats (Go All Out, Push Ahead). Potentially cute on Adam Warlock (extra ready from Warlock's Cape, and can refill the extra HP with Soul World).
As a short term "close out the game in a hurry" option, it's...ok, I guess? The HP will probably tank two villain hits plus maybe a minion, and you're staying in hero form so maybe you can ignore the main and side schemes and just push damage on the villain with your boosted ATK and +1 hand size. I think most decks would do better with a 2 or 3 cost attack, though, as it'll be about as effective at reducing the villain's HP, while also being a useful play earlier in the game.
Note that it's not "just" an extra encounter card, it's another encounter card that can combo with the other encounter cards already being dealt out. Sure, any random minion is not that big of a threat, but when that player flips over a second card and it's Gang-Up, now they're in real trouble (if not outright defeated).
I think the biggest problem is that Symbiote Suit effectively gets worse as scenarios get harder. When Rhino flips Hard to Keep Down or "I'm Tough", eh, who cares. But pretty much everything in Venom Goblin's deck hurts.
Could be slightly better the more players are in the game, as it's more likely that someone will have AOE damage/thwarting, to clean up the extra messes you're generating at a low cost. Just watch out that the extra encounter cards going around the table don't knock out your friends that don't have the buffer of extra HP.