I think it's a good house-rule to have, as I feel that multi-classing is penalized too much with the current rules. But it should be pointed out that Fighters and Rogues do gain ASI's at level that all the other classes don't gain any at, in those cases they should be "bonus ASI's".
The only time MCing was arguably better-balanced, was the one time it was clearly under-powered, yeah.It's the most balanced multi-classing the game has ever had
No, those dead spots were cleared out for ASIs. It's not like no class had ever gotten stuff at 4th level before.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..
I believe that is exactly correct.I believe the PHB is explicitly clear that your ASI / Feats are tied only to your class levels. Thus a multi-classed Fighter 3 / Rogue 3 has zero ASI's / Feats (assuming no Variant Human feat).
That depends on how you change the rule, would be a better way of putting that.That said, I know of at least one DM who is purposefully "misinterpreting" this rule to allow ASI's and Feats at Character levels 4, 8, 12, 16, and 19. "That depends on how you interpret the rule," she says.
Multiclassing is already strong, and becomes stronger. One of the few downsides is obviated completely.Given all this, are there any unforeseen pitfalls that you'd expect from playing the game this way?
Some classes get ASIs at a rate faster than one per four levels. So the result would have been ASIs for character level and ASIs for class level. Seems unnecessary. The current system is fine: multiclassing is strong enough as is.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.
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.
No, those dead spots were cleared out for ASIs. It's not like no class had ever gotten stuff at 4th level before.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.