Silk and Gypsy have true legato for some instruments, whereas Ethno World only has scripted legato. Additionally, I think the EW products have multiple mic positions, whereas Ethno World doesn't. Ra is definitely the shallowest in terms in sampling, since it's from 2009.
Take that how you will. Maybe they're lower quality despite having deeper sampling. I haven't used them so I can't say.
However, I don't think Ethno World is that bad. They went for quantity over quality - if you think of it like a big collection of character instruments, it's kind of nice to have around. I've gotten some really good sounds from it, even if the instruments aren't good enough to do detailed extensive work with.