Lyxen
Great Old One
I have no problem with monsters using exception-based design, but this creates a real problem of narration.
A spellcaster type NPC who has a magical fiery explosion ability feels like they should be vulnerable to counterspell. It fits the tropes of both what the spell does and what the NPC is doing. How do I create a narrative around a spellcaster using an ability that isn't able to be countered? Is it some kind of wand? Is it a subtle spell?
I'm totally on-board with making NPC stat blocks easier to run, but I'm not a fan of creating an environment where only some NPC spellcaster abilities can be countered, and it isn't obvious in the story why some abilities can be countered and some cannot.
And a lot of PC powers cannot be countered either. You cannot counter a bard's inspiration, a cleric's Channel Divinity, a Druid's Wild Shape, although they resemble some spells...