I got the book for my birthday and in some ways I have to agree.
I mean... Having the older versions of the modules are a good thing IMO. There is only one version of the KOTB PDF officially available and in its current incarnation it's not a good one (formatting error and missing sections), which is odd because the PDF that was available 2 years ago when I bought mine was fully functional. So having these good reprints is useful, even if the book is such a brick that it'd be hard to use when running at the table.
Now the 5E conversion.... Yeah. It's not good. It's schizophrenic and seems to ignore the changes in design that D&D went through between Basic D&D and 5th Edition...
What do I mean by schizophrenic? Well.... One of the big appeals of Keep on the Borderlands, indeed what made it a lasting module is that it's so light on story that DMs can truly make it their own. At best a handful of NPCs have a bit of personality traits explained.
Here... in a lot of places this is preserved but then they arbitrarily change things... like the elven advisor to the Castellan suddenly gets a name, a Chaotic Evil alignment and a whoooooooole lot of backstory and lore tying him down. And the wilderness encounter tables were wiped entirely in favor of far more specific encounters with so much story packed to them, as opposed to just an encounter with basic goals for the NPCs explained and the DM gets to do his thing.
This is also the problem with all the expanded areas, they all have very very specific stories tied to them, so instead of "a band of kobolds unassociated with the ones in the caves of chaos, led by a particularly strong kobold armed and armored better than the others, who will instruct his warband to fight to the death if need be" you get "A band of kobolds with very specific goals within the area and probably specific ties to NPCs of other locales and also their leader is the son of Torgo The Eye-Gouger."
As for factoring in the evolution of the game... yeah, it is rather sadly ignored. All the armor and arms are just carried over from how they were originally, where your only options for armor WERE leather, chainmail and platemail... It made sense back then for the light infantry of the Keep to wear leather and the cavalry to wear chainmail, but now in 5E with the light armors and medium armors and heavy armors this doesn't translate as well, you don't go from AC6 in leather armor and shield to AC5 in chainmail to AC4 in chainmail and shield (and so forth) in 5th edition... The same applies to other encounter areas as well. A bit more thought in how armors have changed and how armor class works in 5E could've done wonders.
These two elements just tell me that the wrong aspects were focused on as they worked on the conversion.
As for the art.... Yeah, a lot of it is bland and forgettable (the only image that stands out to me is one of the goblins checking one of their stolen barrels of goods while hobgoblin eyes spy on them from the secret door), and there's also some really baffling design choices.
The wizard who shows up in all party pics, with his "shirt" pockets and bandolier of pouches, ends up just looking like Duke from GI Joe in a silly wizard's hat.
The Minotaur art piece is definitely one of the worst. Despite furnishing the lair with macabre skulls as the module describes, the fact that the savage minotaur in a chain mail coat is now wearing nice pants tucked into cuffed boots just takes away so much of the threatening look it once sported.
It's a nice piece of novelty to have if you're a fan of the modules (and I am, of KOTB at least) but in terms of being useful in play the book is too cumbersome, and the conversion too problematic...