What's wrong with it? I use spotify all the time, have no problems, and can't think of anything I would change. I guess the default recommendations are not great, but they're easy to ignore, and that's not really a software issue.
It's not quite enough to make me use a different service, but it's annoying not being able to just pick up my phone and continue where I was on my computer
The most egregious and the one I can fathom the least is that the podcast feature with video podcasts is just fundamentally broken. On android it is constantly stopping and restarting podcasts, replaying ads, and stopping for no reason. I pay for premium which means I'm supposed to be able to skip ads, but they make it difficult by forcing you to drag the playhead all of the way through the progress bar, for every ad. About 1/10th of the time I do this it sends me back to the beginning of the podcast and I have to go through and skip all of the ads again, sometimes succeeding and sometimes getting sent back to the start again.
This feature also forces you to use video. There is no audio only option. This is obviously terrible if you have a bad connection. I live in a city where the cellular connection is spotty in a lot of places so many times I will want to play something, have it freeze on me over and over again and I will just give up.
There are tons of people complaining about the ad issue and this all over the internet. Spotify is spending billions of dollars developing and buying exclusive podcasts only to have them on a player that is such shit that people who have been listening to them for years give up.
They must have an engineering budget in the hundreds of millions of dollars a year range, and they can't fix these basic issues. There are many podcast apps that are much much better that are available for free. Video sucks on mobile, especially android, but for the love of god make it optional, and fix these basic bugs. I have a very widely used android phone running the latest operating system. I can't imagine how issues like this could exist for years on a company like this' core product without getting fixed.
For another example that I experienced today on desktop mac osx, there are lots of times the player will simply stop, when my connection is on very fast wifi, and I will restart it, only for it to stop again. It is like kick starting a motorcycle sometimes. I think this has to do with bluetooth, but I've never encountered this problem with any other audio or video software. This is something that I have been experiencing for years, on different osx devices, with different versions of spotify, and different bluetooth devices. It is clearly just a very common bug. It is something that should be easily caught by telemetry but spotify is too incompetent or crazy politically to fix it.