I think there are three main reasons why Frantic Brute (FB here on out) is still hated:
1. It doesn’t feel fair
2. You generally have no recourse against it
3. The builds that use it are annoying
On the first point, despite its large damage spread, FB still hits hard on average (432 avg dmg). Low rolls of course exist (it should be noted that a low roll is only 47 dmg less than a Nightfall low roll), but the high rolls are still quite problematic. With its highest roll, FB hits for 438 through full phys res (180); by comparison, Nightfall would hit the same number on its high roll only if the opponent has 1 phys res. Nightfall of course has an advantage in the long run, but a dual frantic setup compensates for this (and if they roll high enough, the long run advantage pretty much disappears). Taking all this in mind, FB is at worst a diet Nightfall and at best effectively negates 180 phys res.
Following from this, the counterplay for FB is only adding more hp and res (assuming the FB mech is rounded), but as we’ve seen previously, FB has a tendency to negate a good portion of res through sheer force. That’s not to say that the res is useless - it just feels that way if the opponent is lucky. With just two FBs (and typically a recoiler), a mech can sustain itself. The only other case of two-use weapons sustaining themselves as the crux of a build is Valiant Sniper, which has counterplay options. Either stay close, preventing the use of both (or all) of them, or bait the opponent into using them up if you have regen and hp + res to tank it (with the Valiants used, most mechs running then will have exploitable range gaps). With FB, it’s just slapping on hp + res and hoping the opponent doesn’t get lucky - there’s no skill or thought involved for both sides.
Lastly, the mechs that run dual FB + recoiler are typically counters. Counters are themselves not fun, so FB just adds on to poor experience.