in BG3 reaction spells like featherfall seem to fire when needed, assuming you remember to cast them earlier in the day ;-) So you have no choice - you may (for whatver reason) not wish to use the spell at that point.... Other reactions, I'm not so sure about.... I thought I had used 'protection' for my fighter, which should be a reaction....if I recall correctly that does impose disadvantage on a nearby attacker of an ally - but you can't choose which ally to spend it on in a turn, which is bad.