Ships that are fast enough and small enough to evade beam's trajectory once it is firing during the beam's very short lifetime also have much less hitpoints. The end result is more often than not, smaller ship still dying faster than bigger one with more hitpoints.
Slashers work by getting two random points in ship's bounding box (I think), so regardless of target's size, the slasher will hit just as well. The only thing that matters here is target's speed, but slashers are unpredictable enough to negate this advantage. As a slasher sweeps in random direction, speed and heading of target might save it from brunt of the beam's impact or it might get hit even more than it would have if it had stayed still or gone the other direction.
I've never tested it, but I'd imagine sum of multiple test scenarios would result in averages where nothing but hitpoints would change survivability against beams. If test scenarios prove that wrong, that's fine, I stand corrected.
While UEF doesn't have beams, Shivans do. And I don't think Tevs would design and commission sufficient number of ships to combat UEF only. And don't forget that it was said that UEF has a ship of their own of same class. Last time I checked, Tevs had beams too.