I would definitely say it's the latency issue. If the speakers are within earshot of each other, the timing differences is likely to make it a bad experience more often than not. If you really want this feature, get a house-speaker system that supports it and have Spotify stream to that _one_.

I a bunch of Denon devices from smart speakers to a soundbar to two home theater receivers and many of them support HEOS which can do whole home streaming.

It sucks that you have to control it with a mobile app (I did write a PC app using Tk but that is not much better than a mobile app!). Also I have had a lot of devices fail either completely or partially (WiFi just went on my 7.1 receiver the other day, the same receiver is relegated to a (awesome) stereo because the HDMI port). The 5.1 receiver was a decontented ‘pandemic special’ with no HEOS.

But that said, the whole home audio is great and works with my jellyfin and Amazon Music and TuneIn and presumably spotify.

probably due to licensing to music labels ...