1. The Prequels were generally objectively garbage as films and most of what RedLetterMedia has said about them is spot on and correct from a film making perspective. There are only a minority of things RLM got wrong or left out of context. Anything positive about the prequels, anything you like is in spite of this fact and theoretical. The execution was overall awful.
2. Most of the 80's/90's Expanded Universe wasn't objectively bad in any way. Most of it was at least okay or tolerable, and some was awesome (Thrawn Trilogy and X-Wing series, for example). It's just that the stuff that was bad was REALLY bad, for example crap like Triclops or whatever. The bulk of the critiques are more in line with "man this doesn't BELONG IN STAR WARS WAHHH!!" as opposed to anything objective. Star Wars was established from basically day 1 as a sci-fi/fantasy hybrid, which would imply to me a diverse amount of things could feasibly exist in this universe without being "out of place". Hence most of those types of critiques are poorly thought out, at best very subjective. On top of that, more generally, novels have less of an audience than films do, and novels continuing on the story of a well known, extremely popular movie franchise just have this spergy nerdy connotation that's easy to write off independent of their actual quality.
3. The EU met all the qualifications to be considered canon, just as I said they were on a medium with a far more limited audience. The rest can be chalked up to George Lucas changing his opinion and story on the EU, regarding it here and disregarding it there, taking from it here and dismissing it there. People who say "the EU was never canon because George Lucas didn't write it" are retarded because by that logic most of Star Trek isn't canon.