Plenty people have pointed out the flaws with this approach, around 3/2/1 and "raid is not a backup". I have hard drives in multiple locations with incremental backups - mirrors are vulnerable to ransomware / cryptoware. Critical information is also in CrashPlan, which is versioned.
The question also asks about experience with storage spaces. Around a year ago I purchased two HGST 4TB drives, formatted them ReFS with all integrity checking / fixing options turned on, and put them into a storage spaces mirror on W10. Since then they've worked fine - just like a single regular hard drive. Performance is fine with my older i7-2600K processor and 16GB RAM, but I don't do anything particularly disk intensive, though I do edit up to 1000 RAW photos at a time and haven't seen a problem. The windows event log seems buggy (maybe because I started on the W10 beta) and I can't actually get any information out of the event viewer, so I have no idea if it's detecting and / or fixing issues in the background, but I can access any of my files all the time so it's likely working fine.
The drive does get very, very fragmented according to Defraggler. The Windows tool defragments somewhat, though it still leaves many fragments. Defraggler can try to defragment it, but it's exceptionally slow. I don't know whether or not it's required.