I don't remember "Islands" but I think I prefer it to "Lizard" (which I also don't remember- only listened to each album once).
Haven't heard "Guacho". Don't think I've heard "The Royal Scam" either. Not much of a Dan fan.
"Sail To The Moon" and "2+2=5" rank among Radiohead's best songs IMO ("Moon" is my # 2 after "Paranoid Android") but yeah the rest is pretty mediocre for them. Still way better than "King of Limbs" or "Moon Shaped Pool" (when will people realize this album sucks? they realized it with "Limbs" immediately and "Pool" is arguably worse) though.
I think "Exile"'s looseness is one of its strongest points, along with its sound (they arguably never sounded better as a band). Sure, the material isn't as strong or focused as it was on the 3 albums that came before it but nothing sounds like unfinished filler to me, even if it is. Obviously not their worst album but of the albums I've heard, I'd say it's about the equal of "Aftermath" (hard to compare) and definitely better than "Some Girls", "Between The Buttons" and *certainly* way better than "Satanic Majesty".