Unless you happen to pick the right adventure, you're going to be expected to absorb a lot of poorly-organised, poorly-presented material, and further, in some cases you'll have to deal with huge and unnecessary gaps in the adventures/campaigns (which again, other RPGs just don't have).
Absolutely agree with everything you are saying, with the caveat that there are exceptions to every rule.
Rifts, for example, IMO had much much worse pre-written adventures than d&d, to the point of being unuseable.
Juicer Uprising: Yes it has an "adventure" in the back of the book, but barely anything is actually statted or written out, it is basically just a written narrative that you are supposed to follow until you reach the end, nevermind if players stray from the adventure (its written as a hard railroad), its all on the GM to prep everything.
Coalition War Campaign: These were possibly the worst of the lot, with a pre-written narrative the players are expected to follow on the rails (and the outcome of the adventure series spanning multiple books is already pre-determined and the players have no way to change it as it is written in the books, the coalition always wins). Significant lack of details and organization, again expecting the GM to do all the work.
By comparison, even some of the worst d&d modules are kinder to GM's in terms of story, guidance, stats, and campaign details.