Kolasinac hasn't had the runs of games a player needs to really establish themselves at a big club, plus the formations have changed a fair bit. He's still fairly young, it would be reasonable to wait to see him play LB for a good run of games in Emery's system before judging what he can really offer.
My problem with him is just that I think he'll get a lot of injuries in his career. What other elite footballer is built like him? I can't think of many. Plus to be an elite fullback these days you need immense stamina and fitness. I just think the sheer size of him is always gonna cause problems with injuries. Bellerin has the ideal build for a fullback.
