I had a look at the backend code - it does support modtime! So I think the overview page is wrong here 
That is true. However if you use it via the local backend then rclone will just read the file content to make hashes, so maybe the smb backend should just do that too? Or maybe optionally?
You are right here. Unfortunately the maintainer of the upstream library has gone quiet.
The consensus from users is that if you can get it connected it works very well and it is quite quick.
However it does not support enterprisey smb shares very well. If you look at the upstream issues you can see that the majority of them are about connecting to enterprisey servers.