I grew up riding cheap MTBs with thumbies, and I've recently downgraded to friction shifting since I damaged my bar-end shifters. There are some ugly but great priced thumb shifters made by sunrace and a couple of other generic ones floating around, not to mention "vintage" ones can usually be run in friction mode, and I found it very easy to friction shift a 9 speed cassette, in fact I much prefer it the click-free shifting.
I suppose grip shifts of quality construction would be the most robust in the case of a crash, but I've always found that twisting motion uncomfortable so I use bar-ends on drop bars on my touring bike. If I ever build up a flat bar bike it'll be friction shifting thumbies for sure.