Psikerlord#
Explorer
If you're the DM, nothing is "overpowered" compared to what you can throw at the party, and you can change the game drastically to make it easier (or more difficult) for your players. If your party isn't tactically minded, they could have a party with every character having "overpowered" builds but nobody being overpowered. On the other hand, they could have mechanically "weak" characters but still laugh at everything you throw at them with a combination of good luck and strong tactical play.
I agree that multiclassing for the sake of minmaxing is a bad idea: even though no builds (that I've seen) utterly break the game, it seems wrong to allow players to multiclass without then having a good RP reason for it. For example, if a wizard wanted to take a level in cleric "because my channel divinity lets me maximize the damage of Chromatic Orb", I'd probably disallow it. If a wizard wanted to take a level in cleric " because he's been a devout worshipper of the god of storms and he's rewarding him for his faithful service", then that's fine. Even if the resulting build is the same, at least one player made an effort to justify it.
For example: to deal with GWM/SS, if you impose disadvantage on the attack roll then if they take the -5 their hit chance is ridiculous. Or a high-AC target that drops their effective to-hit at or below zero: the Shield and Shield of Faith spells come to mind here.
I've only seen crossbow expert come into play once, and when it did it was pretty powerful... However, not having disadvantage for shooting in melee was less of an issue than the fact that the character rolled a +5 to Dex at level 1 so he had two attacks at +7 to hit each and a total of 18 (1d8+1d6+10) damage. Our ranger also hadn't gotten his multiattack yet, and once he got that his damage became competitive with what the CE used to do (before the player left).
Passive perception is a bit wonky, but if you give a clue when a PC detects something with passive perception rather than a full description ("You see a goblin hiding in the woods" compared to "You hear the rustling of leaves off to your side, and see what looks to be a pair of eyes glinting behind some foliage") it can work fine.
To deal with a moon druid (mentioned in every thread about OP builds...), throw situations at them where Wildshape would have to be used in a non-combat role: for example, a sheer cliff where they'd need to turn into a Giant Spider and ferry the rest of the party, or if they need to crawl through the bars of a cage to release the party from a trap.
On the moon druid issue, I find using injuries helps prop up the threat/danger of combats, despite their huge HP pools (albeit I use an expanded injuries & setbacks table, fleshing out the DMG table).