It will work if you have <64 GB of data to back up. You can optionally exclude things that take up a lot of space like your Applications folder. Honestly though, your best bet would probably be to use a "real" drive for time machine backups and/or a third party online backup solution. Backblaze suits my needs quite well.

Now there's an interesting idea. 64gb is plenty for me to do a Time Machine backup of essentials. If that slot is bootable, I'd definitely consider it - an always-available easily-removed backup that you can stick in another machine without extra tools / power / etc.

The OS treats SD cards the same as any other removal drives, so you can choose it for Time Machine. Their site says microSD cards currently only go up to 64GB, so it is of limited use until larger cards become available.