The XCR's downsides to me are:
1) Factory trigger sucks, fixable today with $50 aftermarket trigger job, and RA states they are designing a new more match-like trigger.
2) Ejector is bolted in; I wish it was permanently mounted. There are plenty of options for addressing this yourself if you want, for very cheap.
3) Rail height over cheekweld with the factory folder is different from an AR, so most standard AR mounts put the optics a little high. This can be fixed today by either getting a CTR stock with a cheek piece, or getting lower than standard optics mounts. RA states they are also working on their own SCAR spec multi-adjustable stock.
Weight? Don't see that as a problem. It feels equal or lighter than an AR with a rail system to me.
Remainder of the ergos? Selector is just like an AR except it only rotates about half as far from safe to fire... Mag catch is exactly the same... bolt catch is very similar to FAL, but is less of a reach, and ambi. It is very fast. I don't know what part of the ergonomics anyone would have a problem with.
Caliber conversions? 6.8SPC kits are in customer hands right now. RA states that 7.62x39 and 6.5 Grendel are in production now, and the main holdup at this point is waiting on subcontracted barrel production. RA states they plan to move barrel production in house so they have more control over the entire barrel situation in the future. You can place your order for any of the conversions on RA's website right now.
FN, last I heard, states that they plan no caliber conversions. ACR, I guess it will be up to Bushmaster if they do them or not. I haven't seen any statements one way or the other.