Any maybe it’s too much. But maybe it also balances. We will never know in this fictitious example because the GPF won’t ever get to test it for balance at all levels.
But i also think that stacking debuffs isn’t a bad thing either. If its two damage reduction ones, hammer a blue mage down then a dragon could cloak or shield and two supershots would be wasted on those towers. If it’s a damage reduction and a damage taken increase (fire turret) same thing.
I think that stacking the buffs/debuffs is what could give lots of different strategy to the game and the restriction of 5 towers to a small / 10 to a long but only have 5 in range usually is what helps bring balance. If we could stack every debuff in the game, yeah there would be a problem adding more, but we have this restriction in place already.