Really? My Boucheron I'll admit was struggling for some time, but the moment he promoted into warrior, he easily became one of my top, if not my top damaging units really quickly. It's also so nice having a backup unit that can equip a longbow, which adds up with my Alcryst on Lucina's ring. It's DLC, but I also found he synergizes really well with the three houses bracelet, as the gambits are so useful and he can proc weapon synergy with both Edelgard and Claude.
Hard agree on Lapis though, I tried so hard to make her work, but she was essentially a melee unit that could rarely survive one round of combat while Diamant's just right there protecting everyone. And then Goldmary joined, and her confidence seems hilariously deserved.
It's actually so sad that non promoted on join units don't have any unique outfits in their base class promotions.