Windows Phone 8 users who updated to Windows 10 Mobile lost some functionality in the process, however Microsoft's continual improvement through frequent updates seems to be quickly moving to restore features that were lost.
Personally one of the most disappointing was the loss of some interaction between Cortana and Groove Music. Specifically the ability to start playing a playlist via voice control.
It's a feature that I use heavily when driving so finding it restored in the latest update to Groove Music which arrived earlier today is great news.
Windows 10 on both desktop and mobile is an ever changing feast, and its a platform that is improving with each new update that reaches customers devices. Neither Apple nor Google seems able to iterate as quickly, even allowing for the maturity of those platforms.
Maybe there's life in the old dog yet.