It's the most balanced multi-classing the game has ever had
The only time MCing was arguably better-balanced, was the one time it was clearly under-powered, yeah.
Thing is, even if, in theory, the character who eventually gets each of his levels divisible by 4 in each of his classes, is not 'getting away' with anything, nor being 'penalized' at the end of it, he's still spending some levels, maybe most of his career, behind the curve, relative to those advancing in one class or in neat blocks of for at a time.
If it were any actual feature well, you're just taking the features you want in the order you want them, level by level, that's how it is. But 5e has a progression, Proficiency permeates it, HD are very important, max slot level & extra attacks and cantrip scaling are all critical to it and so are stats - and all those things advance on character level, except ASIs and Extra Attack, they advance on class level.
It's the same problem that plagued 3e MCing in multple areas, but /particularly/ in spellcasting. 5e neatly fixed the problem for casting - spell slots, spell save DCs, cantrip attack & damage, all scale with character level. That is vast improvement over the kludged together caster-level PrCs that 3e used to compensate for how badly it bungled MCing casters together. Yet, while 3e neatly handled BAB & itterative attacks stacking together, 5e botches Extra Attack the same way 3e did caster level.
It's perplexing, because it wouldn't have been so hard to stick to their guns and make the 1/4-level ASI run on character level, and arrange class features to suit from the ground up.
There is a clear level-by-level progression in character ability, ASI's were planned for specific 'dead spots' where the characters are otherwise flat..
No, those dead spots were cleared out for ASIs. It's not like no class had ever gotten stuff at 4th level before.