I honestly like Z-Moves more than I like Megas.
Z-Moves are a one-time super nuke which, I dunno, I've always felt like it feels like a natural progression in a game where you have moves that have critical hits.
Megas though are just.... bleh. There are so many Pokemon who didn't need theme that only got them because they were popular, and soo many popular Pokemon that didn't get them at all. I hate saying it but Megas really are the closest to Digimon that Pokemon's ever become.
If they were more balanced but, usually it's just "SLAP 100 EXTRA BST ON EM AND PUT EM IN THEIR BEST STATS"
Agreed for the most part. Z-Moves get a lot of flack, but in terms of execution and what it set out to do it did its job better then Mega Evolution did. It allowed every single Pokemon access to the moves so any Pokemon could be used with the right set-up and there wasn't as much restriction to it. Additionally, since every Pokémon had access to all except a handful the moves were in general more balanced as you didn't have any that truly centralized the metagame around them, and even the special ones didn't break the game in half. Finally, in terms of implementation into the games it was fleshed out better with a clear origin (their power originates from the portals), integration into the plot (The Z-Ring, Z-Power, and the gems all of that plays into the character's journey), and there's easier access to them for the player as most are made available as you play through the main story. There was a more consistent integration of the concept into the game and its gameplay, and it allowed it to fit into both without it feeling haphazard.
For the reasons you said, I can't say the same with Megas. As you said, a number of Pokemon that could've used it for the power boost/updates to their designs were ignored for popular favorites, and in terms of gameplay there were clear balancing issues as some had abilites and stats that were metagame centralizing (Salamence) while others were very undertuned and couldn't even carve a niche (Audino). The execution made the good concept behind Megas fall short of the potential it had, and it could've used another go through the ironing process to even out the power creep of the forms while also better distributing them across a better variety of options.