From playing in the grand finals of multiple Brawl+ tournaments I can say this for sure:
-The largest defensive problems are airdodging and shield drop time
-Spotdodge and rolldodge are not quite as bad, but still overpowered
I would go into a lot of detail about Brawl+'s imbalance with defensive options but I think we're all already quite aware of that.
Airdodging: One major problem is that using a short-hopped airdodge to approach is too useful. Now, of course it can be punished when they land, probably 80% of the time successfully, but only if you see it coming. And if you're playing tense spacing wars between the characters' strongest approach moves, suddenly facing an airdodging opponent instead of a spaced aerial can be to quick a change to react to, as you have to make some kind of move to counter the aerial that may or may not have come out instead. This may seem like a flimsy concept but this kind of battle was basically the entire spacing in the last grand finals I won.
Furthermore, it's also like we've said before too strong a recovery or safe-fall option. I am reasonably confident that this can be well-solved by either increasing the lag and decreasing the invulnerability (directly changing its power by requiring that much less reaction speed by the punishing player) or by implementing the Melee AD, which I was, like everyone, completely opposed to in early stages of B+ but now I see as possibly a necessity in the future. But that's for a different discussion.
Spotdodge: To be honest, I rarely use this, but I can tell what the power would be like and I think the best fix for now would be, like with the AD, to decrease invincibility and increase vulnerability lag.
Rolldodge: Same as spotdodge.
Shield Release Time: Even though it has been slowed down already it does seem too safe to simply attack out of your shield, not being pressured to only a few options like in Melee (jump, shffl aerial, grab, up b, roll, spotdodge, up smash, jc shine). I dunno, I don't see any harm in approaching Melee's lag frames for this, it does need to be nerfed.