I don't think Guts' iron arm would get in the way of wearing due to it's size being equivalent to his real arm. Also when has Guts every really had the upper hand against an apostle? He always has problems with every apostle he fights until he finds their weak point, which takes time. That's how he fights, he can't easily kill apostles like skull knight or Zodd so he has to basically weather an asswhooping for a while before he can see an opening or find a way to kill the apostles. He are smawt.