Apart from lesser edge holding, does this steel have any other disadvantages from better steels?

It galls a little more than some stainless when machining or working bearing surfaces. It's also a bit harder to weld on.

420HC can be an excellent steel with proper heat treat.

About the best 420HC I've dealt with is from Buck, Paul Bos oversees their heat treat and gets the best out of 420HC.

Well, not sure excellent is the right word... Even with the best heat treatment 420HC still tends to be outperformed considerably in terms of edge holding by higher grade cutlery steels. That said, it will make a decent blade when ehat treated properly. It's just that there are quite a few better (but more expensive) choices available.