Comparing Kyurem-B/W to Kyurem and Reshiram/Zekrom is pretty revealing. On one hand, Kyurem wasn't successful because of its poor movepool, lack of strong coverage moves, typing, and inability to boost. Kyurem also has redundant offensive coverage. Reshiram and Zekrom still have much better STAB coverage and signature moves than their Kyurem-fused counterparts, as well as secondary typings that are (arguably) not as defensively weak. Kyurem-B/W has some scary-ass stats but it really suffers from not inheriting anything truly useful from Reshiram/Zekrom's movepools. It also keeps that Dragon-Ice typing and has no way of boosting its stats so really the offensive power you're looking at isn't that much worse than Garchomp/Salamence/Haxorus with a Life Orb or Expert Belt.
Stats alone won't push these guys to ubers, I think what's going to do it is the Ice-Electric/Fire-Dragon coverage respectively, and the power of their STAB dragon moves. But once they get to Ubers I think they'll find themselves suffering in much the same way Kyurem did; Poorer secondary typing and signature moves compared to Reshiram/Zekrom/Palkia, far less versatile and bulky than Dialga, no priority like Rayquaza, and devoid of any way to boost its stats, which all the other uber dragons have (even Reshiram at least has Flame Charge).