yes, sub par or lower end drive trains will break when shifting under loads like in a climb. That is why they make heavier duty gear just for those folks who want to shift under load.
I have Dura Ace/Ultegra on my roadie and has never failed me even when doing a lot of shifting on extreme climbs in the rockies this spring and a not so extreme climb up mt. lemmon in Tucson this summer. For a mtb I would go with a Dura ace chain and xtr cassette and crank and of course the rear dr.