As an owner of both the WXA-50 and the WiiM Mini, I can attest that the differences between Yamaha MusicCast app and WiiM Home app are significant and lean positively towards the WiiM Home, at least for me. It's simply a much more potent and modern app, by any account.
I like the sound of my WXA and it's really far beyond whatever the WiiM Mini could achieve on its own. But, the Mini connected to a decent external DAC changes the picture completely, and still for significantly less money than is asked for a WXA-50.
I even connected the Mini to my WXA through SPDIF, just to try it out through the same DAC. Result was a coherent, balanced sound which I liked it a bit more than the Yamaha own streamer's sound.
Don't know how WiiM Amp will sound, but if it is to live up to the hype, it shouldn't be a disappointment.