I still have my Lumia 730 and it works great. I switched to Galaxy S5 as my mother got S8 last year if I'm not mistaken, but I still use Lumia once the battery gets low on my S5.
The UI was great, the phone wasn't hard to use, was fast, hardware was way below Androids at the time (I believe my Lumia has 1GB of RAM and Quad-core 1.2 GHz) but it worked perfectly. Just felt good to use it overall.
Android and iOS go with icons like you'd see on computers, but WP had a different approach. You had the home which was made of blocks which you could resize to your liking (some apps even changed what was being displayed on the block). The other way was the list where you just scrolled through it, it was in ascending order and grouped, you could just select a letter to go to that group of apps.
Only "popular" apps the majority would use were Facebook, Messenger, Viber and I can't remember if it had WhatsApp, probably did. That's why it failed.
Microsoft did have a way to port Android apps to WPs, but I believe Android (Google) was strongly against this. Can't remember the name of the project, but they had it.