For one thing, Moorcock is very chary of planned series of that nature; while not denying them the possibility of artistic worth, he is more than aware that the majority of such things quickly degenerate into repetitiousness and inflated bombast... if they were ever anything else to begin with. "World building", per se, interests Moorcock little, as he indicates in his article. It is an aspect of the misplaced pedanticism so prevalent in much of sf and fantasy.
For another, such things tend to simply rehash simplistic views of things, rather than coming to terms with the moral complexities of the issues they (supposedly) revolve around. The appearance of moral complexity in this sort of work tends, more often than not, to be just that -- appearance; it lacks genuine substance and is very much surface, following stereotyped, flat handlings of such matters. It presses button "A" to get the response "relevancy", and button "C" to get the response "deeply philosophical", rather than genuinely requiring either the writer or reader to participate in examining any of these things in a meaningful fashion. Their internal struggles are, by and large, simply the same thing we've seen countless times before, in slightly different dress. They aren't organic -- that is, they don't emerge from the internal growth of the story or the characters -- they are calculated and false. They are hackneyed and have been done before, usually better.
And, simply, the majority of them are just gawdawfully written. The prose is serviceable at best, often meandering and vague, frequently confused, full of solicisms and poor constructions, hackneyed (again!) structures and tropes, and just general, all-around slipshod craftsmanship. Le Guin, whatever her faults may be at times, is most often a very careful craftsperson, conscientious with what she puts on paper. Such was not the case with, for instance, Richard Jordan. To put it bluntly, Jordan was a hack, plain and simple. That he had some innate talent, I will not deny; but that it ever resulted in anything of genuine worth is extremely dubious, to say the least.
(I mean no offense to you personally, but I've read far too much to have much patience with the likes of Jordan, I'm afraid.)