Of course, by limiting all movement to the movement phase, you invalidate rules like Battle Focus, all Jet Packs, and so on. The Tau, in particular, would be devastated by that change.
And, again, I would insist that having every unit move in a unique fashion is *no different* than what we have now and is potentially even more complex. The only units that are too complex in the movement phase are Warp Spiders and, arguably, all Eldar Jetbikes. Nothing else is that big of a deal.
It *would* be nice if we could just stop having to roll for jetpack thrust moves, as that is needlessly time-consuming, but I really, honestly have no idea what everybody is complaining about. If the alternative to having all units move differently is to have all units have a different move statistic, it's the same damned thing (assuming, of course, we don't Sigmarize 40k to hell and back).