I play almost exclusively Mystery Heroes now, unless my friends are on (they’re not such big fans of MH so we tend to play more Quick Play or occasionally 3v3, though I’m less into the latter).
I like playing as all the heroes, and in Quick Play I tend to find myself fitting in with what the team needs most of the time (and I often feel like I have to pick who I’m best at from the suitable options, too). With MH I can really practise as everyone.
In addition to that, there’s an interesting level of added strategy because committing to a kill in MH can be so much more significant. Killing a hero in MH resets that player’s ult charge (even if they roll the same character upon respawn), so taking down Mercy before she gets her ult - at least in her current state - becomes an essential task. Similarly dying to take out a hero who is very strong in the current scenario can be super worth it. Removing their Bastion might be worth several heroes on your side dying, for example, because then they don’t have that character to defend with.