It's reliable except when it's not. I'm using Mojave, and currently fighting a bug where a local snapshot gets stuck. When I list the local snapshots, I see the old one, then a gap of several days, and then additional snapshots.

From what I can tell, this snapshot is preventing space reclamation. The last month or so, I've constantly run out of disk space even when not doing anything special. As in actually run out of disk space — apps start to become unresponsive or crash, and I get warning boxes about low disk space. When you run low, the OS is supposed to reclaim the space used by snapshots, but I guess it doesn't happen,

The stuck snapshot can't be deleted with tmutil. I get a generic "failed to delete" error. The snapshot is actually mounted by the backup daemon, but unmount also fails. The only solution I've found is to reboot. Then I get 200-300GB back and the cycle starts again, with snapshots getting stuck again.

I'm considering updating to Tahoe just because there's a chance they fixed it in that release.