If you fail LMS you dissappear completely without any chance to regroup.
According to P49 of the BBB, if you fail your LMS check you begin to Fall Back.
And with ATSKNF, you automatically pass LD checks to regroup, are allowed to regroup if you're under 50% (but not if out of cohesion or if an enemy is within 6"), and you can move normally
So, for all intents and purposes, doesn't this make Marines immune to LMS?
Seriously, you pass your LMS, you move and fight nomrally. You fail, you begin to fall back. This is determined at the start of your turn.
Now, at the beginning of your movement phase, a unit that's Falling Back gets to try to regroup, (before they make their fallback move). So if you automatically regroup and are allowed to move normally afterwards, then the end result of failing your LMS check is...getting to move and shoot nomrally.
So, come to think of it, LMS isn;t a big deal.
Whoopsie. It seems the SM codex (and the BA codex) explain that in addition to the regular ATSKNF rules, that SM are still subject to LMS, and that they regroup *after* their fall-back move. But it doesn't say that you're not allowed to move after your fall-back move (since regrouping happens at the start of your move phase, presumably your fallback move happens before that), so you can try to recover the ground you lost
So a solo biker who fails his LD test would fall back 3d6" (mathematical average of about 11.5"), and then regroup and move 12" (or turbo-boost), presumably back the way he fell back.
So LMS IS a problem, but mainly in that you're going to be burning all your movement just getting back to where you started.