Time machine is glacially slow, and it is finicky to use with network storage, and doesn't play nice with non-macs. Newer portable Macs had been annoying to use with most existing external storage until rather recently because of Type C.
I have to disagree on all points. If you leave it on, you don't have to worry about its speed because it's always up-to-date. The first backup may be slow but you shouldn't be waiting until an emergency to do that anyways. I have also had 0 issues backing it up to my Synology. I literally enabled Time Machine on the Synology, enabled it on the Mac, and I've never had to worry about it again.