UngeheuerLich
Legend
The worst thing about working out Challenge Rating is the proficiency Bonus is keyed of it, so you end up going round in a circle trying to pin down the exact numbers.
I was worried first too.
Especially npc classes seem to have that issue: 5HD fighter with mutliattack. Still CR 1, and a +2 proficiency bonus, but using multiattack.
I rationalized it that way: PC classes are special and have a special progression od prof bonuse tied to the HD. Other monsters don´t. A Lich even has a higher proficiency bonus than its HD suggest.
For monsters it is easier: why should a high hp but dumb massive ogre should have an especially high prof bonus? Why should a brown bear or an elephant?
Another difficult thing is: If you add say 3 wizard levels to a rakshasa, will it even change the CR? CR is just not granular enough, when 9 levels of wizard seem to be only 6 CR (again NPC).
I wished CR and XP were not too connected, and you could just increase one and lower the other. The former only being an indicator how dangerous it is individually (resitances, special attack, special counter measure) and when a PC should encounter it first, the latter how dangerous it is in a group.
Take the intellect devourer:
in my opinion, the xp seems about right, but CR should be lvl 9 or so, because as it stands now, someone incapacitated by a devourer can´t recover without greater restoration. And will be easily killed, depending how you interpret "protection from evil" working. (Does it restore the brain?)