On another note, since we know that MegaEvos have to be done manually, it means that some Pokes can't use their MegaEvo advantages when switching in the first time. Some that are notable are Absol and Ampharos. Absol can't bounce effects when he comes in, which is actually a bit huge since because your opponent will know about this and successfully get any effect off, such as Stealth Rocks. Ampharos can't utilize his Dragon typing on the first switch in, which might hurt his ability to tank.
Another small thing to note is that each MegaPoke has to get one attack off in order to MegaEvo. If a Poke is constantly forced to switch out (be it from your opponent's momentum moves such as U-Turn/Volt Switch), then they might never get to MegaEvo to utilize their increased stats. This situation is rare, of course, but it's still a possibility and something to take note of.