I'm not sure any single one if those was necessarily problematic so much as the fact they did all of transitions at the same time and perhaps didn't do enough diligence in testing and slowly rolling out to ensure parity. For instance they could have rolled out mdns support with fallback to upnp and perhaps iterated untill they knew for a fact mdns was finding all the same devices as upnp with similar or better latency. However it seems that's not what they did.

mDNS is used by all the major players in the iot space today and there is a reason for it. For instance I believe Chromecast uses mDNS for discovery. Routers have had a long time to work through any possible issues. New code will always suffer from bugs and I'm pretty sure that's been one of the problems they face more than anything else.