I got 780s for France a couple of years back on the big bike. even for the wide open trails they felt too wide. through the mega steep tree filled trails there was a constant threat of clippage which worried me.
they are now at 740 and they feel good for DHing but too wide for anything natural and with trees.
on the XC bike I have good old 685mm Eastons which feel as perfect as they ever have done. one of the main trails I ride on (alongside Wolvers Lane on the way up to Leith Hill) has barbed wire on one side and holly bushes on the other for about 1/2 mile and even with 685s I’m scared of catching my hands.
I’ve got some of those cheap purple 725 Race Face Atlas bars on another bike which feel too wide, but I need the width as that bike has a Weeride baby seat on the cross bar 😀
and agree with rwc03, climbing is horrible on anything wider than 700 – I end up putting my hands right next to the stem with wide bars.