My biggest complaint with Apple Music was the inability to add tracks to the 'My Music' section. I've been banging my head against a wall for a week now trying to figure out exactly why when I click either the + or the 'Add To My Music Library' buttons seemingly nothing happened.
After some intense Googling tonight I found a Reddit thread that answered all of my questions...
I knew as a long time subscriber to iTunes Match there was a 25,000 song limit. Once your library reached 25,000 songs, Match simply stopped working. Tracks wouldn't be available to other Macs or iOS devices. What I didn't realize though was that Apple Music tracks also count toward that 25,000 song limit. In other words, I'd reached 25k tracks, so attempting to add anything from the Apple Music streaming library did absolutely nothing.
While this all makes sense now, Apple didn't do a very good job, or make any attempt at all really, of explaining this from the outset. It's somewhat comforting to know 99.9% of my Apple Music issues were my own fault, but a little documentation would've saved a week's worth of headaches (and several bitchy blog posts).