The Night Herald cultists at the Moonmere have a spell save DC of 22. And while it's been noticed that certain enemy stats are arbitrarily inflated to hit target numbers, that's not true of the save DC which appears to just be the enemy level of 8 plus the Cha bonus of 4 (plus 10). In contrast, the cultists' to-hit bonus of +17 with the crossbows has been seemingly inflated, since the +17 is much higher than lvl+dex+prof+potency.
The result is that the cultists were, at least in my run of the adventure, much more reliable as archers rather than as casters. More attacks per round, larger 'pool' of attacks (30 bolts vs handful of spells), and higher to-hit odds even on iterative attacks. The only tradeoff was the crossbows' moderate damage (2d8+2d6) versus the spells' battlefield control.
PC Numbers:
Barbarian: F +16, R +14, W +13; AC 26
Monk: F +13, R +14, W +15; AC 24
Cleric: F +16, R +12, W +16; AC 28
The cultists didn't have any spells which targeted Reflex saves, so the lowest save bonus was effectively a +13, which means the players had anywhere between 60%-75% chance of saving against any given spell.
In contrast, the odds of not taking a hit from a initial crossbow bolt ranged between 30%-50%. (the iterative bolt matched the spellcasting odds, at 55%-75%)
These are just the base numbers, not accounting for any magic or conditions. However, while there are options for decreasing an opponent's save numbers, most such options allow an unmodified save first, and still don't bring the above percentages much closer to one another. In contrast, there are multiple options for boosting one's own to-hit or lowering an enemy's AC without allowing them to make a roll, e.g. Heroism, true strike, flanking, etc..
These numbers can go both ways; the cleric's spell DC of 23 has somewhere between a 50-60% chance of success against 1 level lower cultists and 40-20% against the 1 level higher dragon, which certainly feels low when I look at it but, as none of players have recently played offensive spellcasters, I can't say how it feels in-game.
I noticed the high odds of saving against magic in Sombrefell Hall, but I thought maybe that was just because, again, the enemy magic was targeting Fort/Will and my players were all armored clerics/paladins. But in Mirrored Moon Enervation was unlikely against any in the party and I couldn't even keep a barbarian confused for more than a round, and where's the fun in that?
I'd like to see Expert/Master Spellcaster happen at much lower levels and allow for other possible lower-level methods of increasing spell save DC's (unless I missing a part of rules, in which case please direct me). I'm not looking to steamroll my players, but I'd like slightly better odds of injecting offensive (damaging and status) magic into combat.