Let's get the flaws out of the way quick - it's not a very good source of exciting rpg combat encounters. No doubt there are thousands of people out there with good memories of amazing battles with these monsters, but those were undoubtedly due to some combination of (1)having a skilled and experienced DM and (2) luck.
I am, at this very moment, looking at the Aboleth entry in the 2nd, 3rd, and 4th edition books (and honestly, this side-by-side comparison of the editions and their various strengths and weaknesses would be a fascinating blog post in its own right) and 3rd edition says it's CR 7, 4th pegs it as a level 17 monster, and 2nd . . .
I don't know. It's got an AC of 4 and 8 hit dice, so a party of 6 1st level characters could likely take it out in 4 rounds. But it's got an extremely deadly damage-over-time attack and the ability to permanently dominate 3 people per day. So I want to say . . . level 6?
It's all just a guessing-game really. But then, perhaps it's unfair to put this all on 2nd edition. Part of the reason they made future editions in the first place was to codify what they've learned about the game over the years. It's a little bit churlish to look at something old and criticize it for being primitive.
But there were other shortcomings that I'd largely confabulated - that the enemies were either boring sacks of hit points or long lists of spells that needed to be cross-referenced from the PHB. Some of the monsters fit those descriptions, but mostly they had 1-3 unique tricks that would be enough to last them the 2-3 rounds they'd be expected to live. Don't get me wrong, I still think every subsequent edition of D&D (with the arguable exception of 5th) managed to improve the mechanical expression of its monsters, but it wasn't quite the wasteland I'd built it up to be in my mind.
On the other side of the coin, neither is the 2nd Edition Monstrous Manual the comprehensive encyclopedia of monsters I remembered. Oh, it tries. Each monster's entry has "habitat and society" and "ecology" sections that, at their best introduce fascinating trivia about the monsters (the Aboleth's happens to be one of the good ones, talking about huge underwater cities built by enchanted human slaves).
But by some weird coincidence most monsters are only as interesting as there was space on the page. Take a mechanically complex monster like a Death Knight, and suddenly there's a lot less room for backstory.
Although, even then, the setting sections of some monsters contain shameless padding, doing things like giving you a year-by-year progression of their offspring's HD or talking about how "there is one 5 HD leader per 30 normal monsters encountered" which seems, on the surface, like worldbuilding, but which mostly winds up being dull as hell.
However, memory fog aside, my grown up assessment is that the Monstrous Manual is a very good book that is held back from genuine greatness by its frustrating inconsistency. It has some real issues with curation. Sometimes, it swings for the fences and comes out with something both unexpected and cool, like the Myconids, a race of fungus-people who can unleash a variety of mind-altering spores when attacked.
Other times, its monsters feel almost obligatory, adding little to the game but taking up space that would be better suited to more charismatic creatures. There's a whole page devoted to sea urchins (including the deadly land sea urchin). Ghost, Haunt, Phantom, Poltergeist, Specter, and Wraith are all separate entries. The yellow dragon literally has no reason to exist:
"Although the existence of yellow dragons has long been predicted by sages (based on theories of primary colors), the first specimen was spotted only five years ago."
Actual quote from the book. What odd setting implications. Who are these sages? Why was the yellow dragon spotted so recently? Who is going out there and documenting new types of dragons?
I mean, obviously, what's going on is that there are green, blue, and red dragons and someone thought "why not yellow," but thanks to D&D's curious conservatism, they couldn't just say "yellow dragons are a type of chromatic dragon, they've existed all along." In any event, that's a page in the book that could have gone to expanding our knowledge of the galeb duhr.
On the balance, though, there's a lot more good than bad, and the Monstrous Manual's spirit of inventiveness is just the right kind of off-putting to inspire a ton of amazing fantasy adventures.
UKSS Contribution: The Monstrous Manual has two different types of intelligent frog-people (three, if you count the Slaadi, but they're more like humanoid frog-demons). The bullywugs are fascist mook-type creatures and the grippli are aloof, long-lived, and wise. Basically frog-orcs vs frog-elves.
I was stunned enough by the inclusion of one type of frog-person, but two is a sign. The grippli and the bullywugs are going to be the same species in UKSS. Some are fascist and some are peaceful, and that's just going to have to be a factor in the world's geopolitics.