Discover Weekly is great. I have a playlist with 800+ songs thanks to it. Unfortunately, though, Spotify fails to shuffle play them correctly. I always get the same 100 songs :(
Same thing happens on all the streaming services I've used. It's infuriating, they always promote certain artists even if you have a playlist with 10k+ songs. I can't think of any other reason (besides saving a few bucks in the distribution, caching side).