A pretty "strange" list, really.
As said before, Ijahman did a few beautiful tunes but so many dull ones ! Yet, he has managed to sustain a loyal following and tours europe on a fairly regular basis. With such a thin amount of solid tunes, it is almost a miracle ! Chalice is in the same league, imho, though they are not concerned with the touring and following bit.
RPO comes as a surprise as well, not that the concept was necessarily dreadful but the production of their LP reeks of 80's smoothness, it sounds VERY dated.
As for Black Slate, yep, Amigo - far from a bad tune, really ! - was an international hit which went up the european charts in the summer after its release. It was a mainstream hit back then, was played often on the big radios and was clearly one of those tunes which did a lot for reggae's appreciation outside the UK. I'd say that, from that same era, Capital Letters and Reggae Regular are clearly underated. I just don't see why getting a big mainstream hit with a good song should belittle a group's achievements. The rest is just snobbery, as if there was a book of reggae rules (one would almost think so these days...). Was that ever reproached to MARLEY ? Hardly.