Are CR 5 undead significantly weaker than CR 5 fish? The level 12 party that I'm running for has just come off of victories against a lich, dragon turtle, and balor; but they were repeatedly near-wiped by groups of CR 5 fish - first by a group of three sahuagin barons riding three giant sharks, and then by five barons with one shark. The combination of HP and sheer volume of attacks made them far scarier than their CR would suggest.
Sounds more like your encounter balance is out of whack. Unless your part is like 7 or 8 people large, or you're only running one or two encounters per long rest, I can't see how they are taking on lichs and balors, unless you just did straight up no-tactics-or-INT kind of fights. In that case, the action economy is heavily skewed towards the players.
As for the fish, baron x3 and gsharks x3, that one encounter is 400 xp shy of being as difficult as a balor. And if you're not running the balor with proper tactics, I can definitely see the fish squad wrecking more havoc simply because they don't *need* tactics to shine. Also, in both cases, you're looking at borderline deadly encounters, assuming your party is at least 5 large.
I'm going to go out on a limb here and say you aren't doing 6-8 encounters per long rest.