This command works sudo omv-rpc -u admin "SMB" "getShareList" '{"start":0,"limit":25}'. Are you passing start and limit? They are required.
No, I didnt even tried that, as they are optional everywhere else. Thanks.
I have already asked HA community to submit feature request and how they want to use it.
May as well make this to a fully featured integration, since I already commited into supporting it further.