Well, mega scizor has the utility of being better for the bulky sword dance set, has its upgraded defenses allow to take more hits, and it still has roost, and bullet punch to deal with his speed issues, you know?
Also, mega blaziken is better, hands down, as due to having 100 speed, it can afford to run an adamant nature much safely, as normal blaziken would have to run jolly in order to outspeed powerful scarfers early on, and also, adamant mega blaziken is stronger than jolly regular blaziken by around 20 points, while the slightly upgraded 80 defenses allow it to take unboosted priority hits a bit better, which is always nice, and while being stronger and faster than an LO jolly blaziken, it can live longer due to laking the life orb recoil, and hitting a bit harder, which gives it the advantage.
Also, mega abomasnow incresed offenses, and defenses, might make it a bit better, think of it, firing strong blizzards, having priority in ice shard, wood hammer, yadda yadda, it isn't that bad really.
Well, venusaur has leech seed, and synthesis, while i don't see it being OU, it can be a good tank in the lower tiers, laking 2 of its weaknesses, while having boosted defenses, and upgraded 122 special attack, it can be very workable.
I agree with you on alakazam and hera, though