Caligae

[Martin Moser]

The hobnails must have been terrifying to be on the wrong side of, but similarly how much protection do caligae offer the wearer?

Not much I'd say. Obviously they didn't see the need for that. Nor for sword arm protection for most of the Roman era. The same is true for their enemies and for many armies both before or after them.
Armor I think always is and was a compromise out of wearability, protection, technical feasability and cost (and maybe more). Most of the time the outcome of this equation leaves room for somebody else to put a swordpoint in ....