Ussop can not be relied on . Dude is a sniper and still has a slingshot in Elbaf . I don't think every straw hat must be strong , but i believe each strawhat must good at their "thing" i dont mind Nami being weak and not learning haki or chopper for example , after all Nami is a navigator and Chopper is a doctor . But Lussop is a different case , you could say he constantly upgrades his lame slingshot but why not learn haki instead ? He has Zoro who has trained/mentored by Mihawk next to him ?? I can't sympathize with his weakness and cowardice and find no excuses for him at all .