Thinking about the Roland ones. Can they be used instead of overheads?
NO.
It would be a nightmare for feedback and for leakage. Omni mics are not usually used for overheads unless they are super close to the source. There was a cymbal micing system that existed that used small omni mics for each cymbal. But its not really used anymore. Omni for overhead means guitar, vocal, and and anything else in the overheads on top of drums. But for recording your own playing it would be great.
Chris