Yeah, Venom strikes me as a bit of a rough transition. I originally picked him up as an alt in AC because I realized I could consistently do 6P-6H-Carcass Raid, but when I saw the changes list, I backed down. If you can do the Dubious Curve combos, it's probably not as big of a deal, but I'm ass at DP's in this game.
6H no longer ground-bouncing doesn't just hurt in combos, but also in block strings where you'd normally stick it out to catch half-screen jump-outs with full combos. My local (former) Venom player also complained about not being able to use the teleport as an evasive tool anymore due to increased start-up, but the recovery was at least reduced so it's better for mixups.
The new FB.Stinger Aim is godlike though, and I can't undersell the utility of the new formations. Overall, I'd say that he's as strong as he ever was in what he does well (transitioning into pressure from zoning), which is what really decides his matchups.
Testament has been turned on his head. Forward EXE Beast is no longer a pseudo-reversal, and his main damage loop is gone. His combos look way more interesting as a result, but I haven't really looked into what they're actually like to perform. The new EXE Beasts kind of act like Dizzy's fish bite (except you control the timing of the bite) and can lead to left/right mixups with his new teleport. FB.Curse is amazing too (homes in, lasts a long time, unblockable, but crouchable). Generally, he just seems more "Testamenty" in this version, so just make sure you're used to managing his traps. Everything else seems so different.
Potemkin has to work a little harder now. My understanding is that the new Slidehead relies on a much more situational FRC follow-up combo, and, yeah, midscreen Potbuster doesn't get a follow-up against the majority of the cast. Corner Potbusters, however.... Those hurt. Every hit he lands right now transitions easily back into his strong oki mixup game, and that's where he really shines in this version, IMO. Coming from AC, his fundamentals will carry over, but you'll have to figure out new combos.
As for Johnny? I believe he just got new, awesome things that do nothing but make AC Johnny players' lives easier. He's still a pretty challenging character to play, don't get me wrong, but his payout for landing a hit has only improved. Mash that up-coin and make it rain!