Hovering over your saddle through uphill chop
This is what I struggled most with when I was running only hardtails for a year (?) or so.
Not actually the technique, but after a couple of climbs when I'd start to fade I found I didn't have the stamina to do this well, so ended up pedalling while seated and going over minor corrogations and steps. This still puts a fair bit of force into the body unfortunately, and I found easy-to-moderate seated climbing the most fatiguing aspect of riding (a hardtail). It made my lower back ache and get stiff, which then starts to effect the descents.
When I was still feeling reasonably fresh I didn't really find any issue with descending even in reasonably rocky terrain. Modern geometry hardtails are still
very capable. Personally I ended up with a short-travel/lightweight dually (don't say the DC word!) as my go-to bike as it does most things except more extreme rocks or speed really well, but without being so punishing on the body.
If you have the option, I would suggest trying to borrow a hardtail and trying it out for a couple of rides. You'll know pretty quickly if your body thinks you should have a hardtail or not, but if you're like me and were a bit borderline to begin with - just be aware the fatigue does gradually build up over time if you're riding regularly.
That's not to say it can't be done/you shouldn't consider it - just don't go in with rose-tinted glasses dreaming of back when you were riding everything on a hardtail without issue when you were 15 or so...