As long as the boot fits your foot I don't doubt the strap system. I had F20's (previous gen F boots) and never had the laces slip. The lace locking system is solid. I would worry a little bit about trying on salomons in the store then ordering a fusion type boot online. Since the F boots don't have a separate liner they fit differently. There is less fudge room with sizing with these then traditional boots with removable liners.
Do you know if run any smaller or bigger in size? I tried on the Salomon faction boots if that helps.