abirdcall
(she/her)
As a debuff to saving throws it's good, but it also potentially becomes a spell slot tax, where you just burn your reaction and an extra spell slot to make the enemy have to make their saving throw a second time. Unless you know the enemy's stats, if they make a save the first time it's hard to know that they are not likely to make it a second time.
If it is a 'spell slot tax' then every spell is a spell slot tax.
Casting Banishment is a 4th level "tax" that burns your action to force the enemy to make a saving throw. I'll take the 1st level reaction spell over that unless you're saying that saving throw spells like Banishment are bad in which case I wholeheartedly disagree.
As for the 2nd part, if you're casting a saving throw spell at an enemy that has a high save against that spell then you've made a bad choice. Just because it is possible to make a bad choice with a spell doesn't mean that the spell is balanced.