I've always had a lot of issues with streaming sticks, and even more issues with smart tvs. Especially getting them to play more difficult codecs from my plex server, or handle scrobbling at a decent pace. The early cheaper/mid-range fire tv sicks have a bad rep, but they seemed to have ironed out the kinks here. Its performant, no screen-tearing or long buffering after ff/reverse. The android app access/selection is excellent. The remote is excellent in range, quality and speed and the voice search is good - although its integration is terrible. It doesn't seem to work inside 95% of apps, even to just enter a search term. Overall, the software UX definately pushes you towards the amazon streaming services, which I very rarely use (i have amazon prime, but mostly I use spotify/twitch/youtube/plex). Its not difficult to install and launch apps, but it defintely seems like an afterthough compared to how google's android/google TV UX works, where apps are first class citizens.
That being said, the apps themselves run great (and you have access the just about everything you could want) due to it being and android TV based software, its just the homescreen and voice search which could incorporate them better. The cast functionality from your phone seems to be available everywhere (although this is more on app developers to implement I believe) and performs way better than my 1st/2nd gen chromecasts (similarly priced at the time), with no disconnections between the phone and the fire stick and fast response to your phone.
The other main drawback other than pushing amazon services is the lack of 4k support. It seems to do a decent job of handling odd encoding like 10bit as well as subtitle formats from my plex server, but its 1080p only. I still don't own a 4k tv (in 2020, outrageous) and I mostly watch Anime where you only get 1080p max, but in the interests of futureproofing and for the odd hollywood movie I watch, if I upgrade my other TVs chromecast, I'm going to get a 4k capable stick. Unfortunately, Amazon don't sell their 4k stick in Australia, so I'm going to probably look at the new Chromecast with Google TV unless Amazon start selling theirs here soon.