Can anyone tell me good reasons to not use SXDC for TM backups?
I think it's a matter of what you backup.
El Cap on my mini is about 58 GB. My accumulated TM backup is currently 206 GB (about 2 yr worth, includes Mavericks and Yosemite along the way).
If you intend a one-time TM, it will take whatever space your current boot drive uses (I think -- I didn't look at it initially).
Is that what you plan?