The "nervous" critique of RRs is a very very common complaint amongst boar hunters who try them out. The word "gutless" is also used. I don't think it's recent poor breeding that has made them that way, I think they're supposed to be like that, like all baying hounds of dangerous game. A bold brave balls to the wall ridgeback hunting lions would just get killed immediately. They have to be extremely wary and being a bit nervous and jittery can actually help to make them focussed on the potential danger in what they're doing and ready to spring back out of harms way at any moment. Boar hunters buy them expecting them to just torpedo into a nasty big boar like a bully grey or something but they're much more careful and wary and can seem "gutless" to the perspective of the hunter.

Like I said it just makes sense given their background, I don't think it's a new "flaw" bred into them. For starters greyhound aren't gutless, they're recklessly brave in fact, don't know their own limitations and get hurt if hunted with pure for this reason. Also I'd maintain that breeds like bullmastiff have been infused into recent RR lines, making them more brave and laid back, and this is why today some of the RRs hunters get for boars do work out. I think if they had access to the original lion hunting stock every single dog they tried would seem way too nervous and gutless for what they expect from it.