I am running quite a few HEOS devices (3 receivers and 8 wireless speakers) and I've been less than happy with functionality over the last few years...
However, a few firmware and app updates have dropped lately, including one today. I didn't find a change log but they are fixing A LOT of the long-term issues. The one today allows me to play the 11 HEOS devices as one system (basically filling the whole house with music.)
Really worth keeping an eye on it if you are a HEOS fan or Denon customer.