Destroids are not behemoths. They are severely underpowered (I am talking about power output) compared with VFs and Regults, and their agility sucks.
FV
That Spartan Hikaru drove in that one episode was agile enough.
'S also one of the few destroids that NEEDS to be particularly agile.
Things like the Phalanx, Monster, and Defender don't have much need for fast movement.
The power diffrence is, IMO, largely because they don't fly or transform.
And a naked destroid tends to pack far heavier weapons and armor than a naked Valkyrie.
That's important.
You will never see a Valk firing a 30 centimeter railgun because the recoil would rip it apart.
By the same token, you will never see a Monster leaping into the middle of a pack of enemies and inflicting pain and suffering on them, because it just can't leap. Well, it might could, but it would almost certainly be seriously damaged on landing. And once it was there, it would be little more than a slow-moving target.
It's sole reason for being is to tote those railguns around. It is a long-range artillery piece. Agility is not needed.
The Spartan is explicitly designed for melee combat. Agility is needed, and it delivers.
The Valk is a jack of all trades. It's fast and nimble enough for melee combat, but lacks enough mass to do a whole lot of damage.