I've never been a fan of multi-classing to begin with because it leads to all the broken builds. WotC has a hard enough time balancing individual classes against each other, and this imbalance just gets exacerbated when multi-classing gets involved.
The majority of MC builds are less powerful than single class builds in the same niche/role.
As a result, some of the design decisions I've seen in One D&D so far seem to be with the intent of curtailing multi-classing abuse. These abuse concerns have weakened potential class abilities that could have been so much better otherwise. For instance, there's nothing inherently wrong with using Proficiency Bonus as an ability limiter since it is steady, consistent increase, as opposed to the sporadic increase of Ability Score Modifiers. But using PB leads to abuse because your PB increases independently of specific classes.
That isn’t even really “abuse”, it just causes the ability to not work as intended when multiclassing.
The fact that the designers are accommodating false concerns of phantom balance issues is a separate problem, IMO. I mean, being a sturdy tiny creature as a Druid is just not OP in any way, and yet they felt the need to gate it at level 11 or whatever like it’s a big deal.
In my list, #1 was an attempt to limit mega-overpowered builds, as the more classes used creates more power imbalance. A good example of this is the Fighter 5/ Rogue 3/ Ranger 3 with Extra Attack and Action Surge, coupled with the Assassin's Assassinate ability and the Gloom Stalker's Dread Ambusher Ability.
This is probably the biggest disconnect, then. My observation is that there aren’t any mega-OP builds in 5e. Even the ones with questionable rules status like the “coffee-lock” aren’t actually
that powerful. D&D hasnt had actually broken player options since 3.5.
#2 was an attempt to avoid the Charisma class abuse (Bard, Paladin, Sorcerer, and/or Warlock) that a lot of multi-classers tend to gravitate toward. It limits mulit-classing to the core 4 classes of Cleric, Fighter, Rogue and Wizard.
Just ban multiclassing, then? Like I don’t know why you’re allowing it at all with restrictions like this. That’s the biggest thing prompting my “but why tho?” Comment.
#3 was a clean and simple way to avoid dipping. If you must multi-class, commit to it fully.
Dipping isn’t a bad thing, though. If the character makes more sense with 2 levels of fighter, but is very much a Valor Bard (assume swords doesn’t work for the character. Plenty of Valor concepts make no sense spending dice to do cooler attacks rather than to bolster allies) it makes no sense to force them to take more levels of fighter.
Nearly all problem builds are theorycraft that “wreck games” at like 2-3 levels of play at most, and are on par or worse compared to standard builds the rest of the time.