Oh for sure and as a reader in the 1990s I can understand that too, because whilst things were improving, you didn't really see a sort of explosion of well-written fantasy until around 2000 and thereafter. Plus in the latter 1980s I imagine a lot of the great 1970s fantasy was out-of-print and only available second-hand or the like - I know Moorcock didn't get republished until the 1990s, and only then thanks to White Wolf of all people. Particularly I think this impacted epic fantasy.
Why that one, specifically? I recall that being the book that started to sour me on Malazan, because it's so truly relentlessly grimdark and occasionally pretentious, that for me, at times it went past compelling or upsetting and into "unintentionally funny". But it still had something - it didn't have Tehol Beddict in it! So perhaps it's the "last good Malazan book"?
Malazan is a series absolutely chock full of characters (and events) that are a particularly immature teenage boy's idea of cool (I presume lifted directly from the RPG campaign the series is loosely based on), but they're most charmingly awful (Anomander Rake and Icarium are generally just funny), and they're layered with more reasonable and relatable characters like the Bridgeburners, and whilst the series teeters on the edge of "accidental self-parody", it's not until the book after The Bonehunters, Reaper's Gale that it finally plunges off that cliff.
And Tehol Beddict is the man who pushes it. He's the most truly obnoxious and unforgivable of the immature ideas of cool - the narcissistic sophomore college student's idea of cool!
Beddict is, of course, lazy, unkempt, rude, utterly lacking in real empathy, decency, or caring for others, but he's "the smartest man alive", inexplicably ripped-as-hell, and supposedly really attractive despite being permanently dishevelled and a jerk (as so many university students have thought they were) and everyone else is just a dumb-dumb next to him, we're told - always told, never shown. Ever. He never does or says anything smart or insightful. His plans are shallow and obvious (always the problem writing an ultra-genius). But via the magic of him being "the smartest" they all go off and we have to endure this character who feels like he escaped from a bad and forgotten 1980s or 1990s college comedy, one unpopular because it was just too on-the-nose re: student ego-stroking.
(There's a whole other angle with Erikson fundamentally undermines his own setting in Reaper's Gale but we'd be here all night if I discussed that.)
Personally I'd have rated whichever earlier one it is that focuses on the undead cavemen as the best, because it genuinely expanded my vision of fantasy and gave me ideas which were actually new-to-me.
I REALLY wanted to like the Malazan series. And I did at first, but then it just collapsed under it's own weight - with all the stuff you mentioned being most of the key factors.