Rolling Beasts are quite possibly the worst legs in the game, but the reasons why will require a bit of explanation.
Reason #1: Phys Meta Builds
For pretty much all of the current phys builds (double shotty, jumper, scope, and Spartan/Frantic), Rolling Beasts either can't be used or actively work against you. In the former case, Rolling Beasts' lack of jumping locks out two powerful combo weapons - Ejection Blast and Distance Shredder. In the latter case, jumping gets you out of the very corner of the arena if your opponent is right in front of you. Stomp + Rock Recoiler deals more damage, but jumping behind them has its use cases (getting out of scope range, getting out of the corner if you somehow burn through the recoiler's uses, getting out of hugger range). Even suspending that, The Claw is more useful (mostly in lower ranks) for its greater HP and comparatively niche mobility drawback.
Reason #2: Little Need for Extra Mobility
3 move space rarely comes into use, more so on phys than any other type.
Reason #3: Outdated Stats
This is a problem that plagues a lot of legs and doesn't need a lot of explanation, as every relevant aspect of these old legs is overshadowed by the Massive Feet series.
Reason #4: Fails as a Sidegrade to Sparked Runners
Unlike Rolling Beasts, Sparked Runners actually do see some use in high ranks on en mechs, namely glass cannon drainers. They're light, drainers don't use jumping weapons, and they have a very niche use in allowing you to get to range 6 from range 9 in one turn. Those last two features are shared by Rolling Beasts, so wouldn't they make for a tankier option? No, not really. Beyond 5 kg of overload, Sparked Runners overtake Rolling Beasts in terms of HP, not to mention that at best you're only getting 87 extra HP. That could be useful, but you need a whopping 20 kg to max the bonus out and a minimum of 15 kg to get an impressive 3 extra HP. That extra weight would be much better used by a hook and/or res drainer.
Proposed Buff: Lightweight and Efficient
Bringing Rolling Beasts more in line with the design of Sparked Runners seems like the most straightforward solution. To put out numbers just for some degree of concreteness, something like 576 HP at 120 kg with res and res drain either being standardized or removed from the Massive Feet series could do (obligatory disclaimer that I'm just throwing about HP and weight stats and can't test it). Not only does the ability to use jumping weapons actually have a downside now, but it also has an HP efficiency second only to The Claw. Lowering the weight does have the potential to kill Sparked Runners, so just a straight HP buff could work so long as it maintains its efficiency. I can't really think of a rework that doesn't go this way, so if you're more creative than me, have at it.