On my MTB I have Shimano Acera shifters which you have to move your thumb through about 100degrees and put in a lot of effort to change gear and they slip. Could be wear and tear but they are not very old.

Was after some recommendations for Shifters that don't destroy your thumbs?

thanks in advance

You'll need to stick with the same brand so the shifter and mech play together.

Upgrade to either SLX or XT and things will help. Shifters arnt usually that much.
Also new gear cables and outers will help remove friction from the shift and make things easier

There's no reason Acera shifters should be so hard to move. We have them on our hire bikes and we've noticed that they are a lot tougher once the cables are worn. It's certainly worth investigating cable routing (make sure there are no sharp bends) and also replacing the cables (inner and outer).

Sram XX1 Eagle.

I can change gears by tipping the shifter with out holding on to the bars.

If you don't fancy spending a sh!t load of money, swap cable and housing. Use a little lube when pulling the cable through. Apply a little grease to help seal the housing. Check for tight bends and if you find any figure a way to give it a larger radius. Open the shifter and give it some love.
Same for the rear derailleur. Make sure it's adjusted correctly, like the b screw. Youtube has tutorials should you not be sure how to adjust it correctly.

Are they actually too stiff or is it more a case of the lever 'throw' being too much for small hands / fingers, meaning you have to contort your fingers in order to push the thing fully home? ... I've come across this quite a lot.

Thanks all

Gear cables etc have been replaced (and greased), couple of times with no improvement,

will give it a total service and check radius see if that helps

Check the pivots are moving freely on the deraileur as well. The rest of the sequence might be fine after all.

Greetz

S.

Always replace full cables and outers as a starting point. Next point would be sticky shifters. Seen a few where the stock grease has dried out/been washed out and they start to stick. Bit of time with some gt85/wd40 type stuff and working the mechanism should free it up.
Given the cost of acera shifters though I'd just replace them and they come with new cables.

you have to move your thumb through about 100 degrees

See, I'm reading that ^ and thinking it's more an issue of lever travel than it is stiff or sticky. It's not uncommon on low end shifters but only generally noticed by those with small hands / fingers. Those sporting shovels under their gloves wouldn't really notice.