"I'm going to apply gameplay conventions to lore"
It is a good question. Lorewise, there are also no examples, that I can recall, of unbreakable barriers. The ones ingame that we need a mage to get past should also count as gameplay conventions.
There is actually an in lore instance, that I can remember (there might be more), of a normal swing breaking a protection spell.
In "The Stolen Throne", Maric was able to shatter Severin's protection spell by putting his full weight behind a blow.
It is, however, worth noting that Severin was being affected by a poison that disrupted his concentration and Maric's blade was enchanted.
Still, the text very clearly speaks of it even leaving cracks in the barrier.
It is possible there are two types of barriers. Mobile and stationary and the first are weaker but easier to cast. For instance, either Corypheus or a group of Venatori mages can cast a barrier capable of withstanding a dozen Pride Demons. That should be the ultimate defence but we do not see Corypheus walking around coated in it.