Tony Vargas
Legend
Arguably, the "route to power" in 3.5 is the single-classed caster and/or full-casting PrC. There's no PrCs in 5e, and full caster level isn't quite as vital with spell DCs scaling with character level and spells scaling with slots (so an MC'd caster/caster isn't as screwed as in 3.x).
Of course, the "route to power" in 5e is simply gaming the DM. It's evocative of the classic game, that way, and if you're going to embrace being all Empowered, you get to put up with that kinda thing.
- Sure, while caster levels (sorta) stack, now, fixing a major issue with the original 3.0 MCing, 5e MCing puts a similar onus on ASIs (in contrast to 3.x, when feats & stat bumps were simply based on character level) and Extra Attack (I presume, the main 5th & 11th level bumps you're thinking about).
The fighter, certainly. While getting 3rd level spells and 6th level spells, respectively, is certainly a bump, it's not like 4th and 5th level spells are anything to sneeze at, though. But, yes, being conscious of the significant breakpoints of the classes you're combining is a good idea, otherwise you may pull in a class to get something and wind up getting it very late, or even missing out on it altogether.
In the sense that they gain abilities at low level that can synergize pretty nicely with other class abilities, sure.
I suppose it could depend on the exact build and the vagaries of the specific campaign, but neither should be feeling to OP compared to a 5th level full caster, anyway. Nothing much to worry about, there.
The balance of 5e is quite loose, that way. At any given level, in any given circumstance, a given MC combo, like a give class, might be wildly (in)effective. It's mostly on the DM to manage that variation to maximize the fun of his campaign.
Under-performing, even in that sense is a concern for the DM, in that he can find ways to keep such a PC involved and contributing, based on whatever his combination of abilities does bring to the party(npi). It's not like 3.5, when such considerations could render an otherwise cool concept non-viable.
3 levels is OK as an arbitrary example, depends on how long the campaign goes. The extreme case would be the multi-multi-classed PC who takes 2 or 3 levels of many classes and /never/ gets an ASI. Kinda like the 3.x MC goof who ended up with +0 in bad save.
Delaying, again, depends on the campaign. One objective guide, though, is the exp it takes to level vs the expected exp of challenges, it indicates that characters generally level very fast the first few levels, then slow down through about 11th. So that first bump normally comes very quickly, if you delay it only a few levels, that may actually be a lot of play time. OTOH, advancement speeds up a little after 11th, so getting a kicker at 13th instead may not be that bad. OTOOH, if the campaign wraps at 15th, getting your 11th level bump at 16th is not getting it, at all.
