Here are the megas that I don't see generally being more useful than their regular forms:
Alakazam — Lack of magic guard is a big loss and it still can't take any physical hit meaning priority ruins it. Plus it doens't hit as hard as LO Ala.
Scizor — The stat that got the biggest boost with M-Scizor was defense while its speed got a measly + 10 and its attack only got +20. Not really worthwhile.
Heracross — Awful speed means it not only does less damage than banded cross but also gets outsped by a lot more pokes.
Blaziken — LO Blaziken is just straight up stronger than mega-Blaziken and speed/SpA isn't really an issue.
Abomasnow — Outside of TR, this thing doesn't have much use
Venusaur — yes, its defenses are slightly better and thick fat helps but now it doesn't get black sludge recovery which is pretty crucial for something to take repeated hits unless it can reach monstrous defenses with an eviolite. This one's a little trickier to judge since Venu w/o chlorophyl is so dissimilar to the sweeper of gen 5.
Everything else seems to have at least one decent reason to be used over its regular form.
Alakazam — Lack of magic guard is a big loss and it still can't take any physical hit meaning priority ruins it. Plus it doens't hit as hard as LO Ala.
Scizor — The stat that got the biggest boost with M-Scizor was defense while its speed got a measly + 10 and its attack only got +20. Not really worthwhile.
Heracross — Awful speed means it not only does less damage than banded cross but also gets outsped by a lot more pokes.
Blaziken — LO Blaziken is just straight up stronger than mega-Blaziken and speed/SpA isn't really an issue.
Abomasnow — Outside of TR, this thing doesn't have much use
Venusaur — yes, its defenses are slightly better and thick fat helps but now it doesn't get black sludge recovery which is pretty crucial for something to take repeated hits unless it can reach monstrous defenses with an eviolite. This one's a little trickier to judge since Venu w/o chlorophyl is so dissimilar to the sweeper of gen 5.
Everything else seems to have at least one decent reason to be used over its regular form.