Ring Man seems quite harder compared to the original. I'm not savvy enough to understand what's different, but at least two things I noticed is the rings not traveling as far when thrown and when they return to him from a different altitude they sort of curve a little bit (in the original they follow a straight diagonal path), both of these make it much harder to dodge them.
Not sure if this is a bug, if it's an intentional buff feel free to ignore.
He's by far the only robot master that felt different to me during a buster-only run, was happily surprised to see Toad Man can still be beaten without moving by timing your shots when he raises his hands and Bright Man still restricting Flash Stopper to the appropriate health values.