Something to keep in mind is that buoyancy sort of goes with fly, accelerate sort of goes with quickening/haste, and petrify flesh sort of goes with hardening/stoneskin.

They're all mixed blessings, moreso with the similar spell in play. So for example you might choose to accelerate someone who comes at you hasted -- it gives them a little more upside even, at the cost of some big drawbacks that you can capitalize on.

Buoyancy lets certain kinds of hits automatically send a victim flying; it also makes some kinds of attacks that care about the mass of the character very hard to do or defend against.

Accelerate ups defenses, but it makes bleeding even harder to deal with than with metabolic quickening alone, makes regen even worse, and makes you burn moves to fight. When you run out of moves, ####, as they say, gets real.

Petrify flesh is DR, at the cost of lagging yourself more while it's going.

Metabolic unity shields you from some of the drawbacks of these combos cast on your self, up to a point. Situationally I would use any of these spells on myself.