Hi @MattSuda,
I did also pick up the topic you mention but I believe it's actually outdated two years later. Spotify has apparently reconsidered its position as I can witness that the W10 Mobile app is working pretty fine on my Nokia Lumia 930. The app version is 5.2.4.0 (the last one I believe) and my system is up-to-date too.
In a short I have been using Deezer Premium service on my PC but their Win 10 Desktop app is just a **bleep** compared to the browser client edition. As this app was remaining too buggy and very poorly featured I decide to give a try to the Windows 10 Desktop version of Spotify. Reading here so many complaints from users about the W10 Mobile edition, I was quite scared to download and give a try to the mobile edition on my Lumia 930. But I did 🙂
And suprisingly I have to claim that the W10 Mobile edition ot the Spotify app has been working pretty good for over one week on my mobile. No issues at all in online or offline modes and I have extensively tested everything: any available feature is working as it should!
I will keep on testing it as I have free trial plan for two months. I hope I will subscribe forever as long as Spotify supports the Windows 10 Mobile platform. And forget Deezer Premium which music catalog is quite similar.
To answer the OP, I am missing two features: first some sort criteria for albums in my library (artist, genre, album title, rating, etc.) and second sync of the current playing queue for the user account in order to be able to play it on Desktop and Mobile when I switch from one to the other. The first topicis discussed in the community here: Re: [Windows Phone][Your Music] Sort/Filter Options for Your Music
Maybe I will add some others in the future! And of course if the app goes wrong upon an future update or just over time, I will report it here!
Cheers from France and congratulations to Spotify that makes a Deezer unsatisfied user happy again with its W10 Mobile music app 🙂