If you switch from road to MTB you may need to alter the width of your BB spindle, ie a new BB cartridge.
Octalink and Isis use wider, stiffer spindes so have to use smaller bearings that wear out more rapidly. I dont notice any improvement using a stiffer spindle.
The current external BB units offer stiffer, lighter spindles, bigger bearings and ease of fitting BUT they are still trying to squeeze lightweight components into a space made for solid steel. At some point manufacturers will change to a new BB standard such as BB30 or similar (hopefully there will be lots of different standards to chose from). Till then the best options are square taper for durablity or external for lightness and performance.

MTB chainrings are smaller so work best with a matched front mech with a tighter curve, BUT road gear levers from Shimano cannot index MTB derailleurs. The solutions are to use:
- Tiagra triple mechs with a short tail. Beware that some versions of the triple mech interfere with the chainstays so cannot be lowered sufficiently.
- Bar end levers that are non indexed.

Campagnolo make shifters that micro-index at the front so work with any mech BUT the rear shifters do not play well with anything but Campy of correct "speed".

If this sounds complicated, its because Mr Shimano doesn't tour.