my dad uses a garmin on both road and MTB.

I used a traditional, albeit wireless, computer on my road and cx for years before the past year or so, I don't bother. I track most rides in a fitness app so I can see where I'm at (and get some motivation for when I haven't been riding enough) but I don't need a cycling computer on singletrack at all. there's usually no time to look down, anyway.