I haven't watched enough tape to say if that's true or not, but it seems like people have been saying that about every single DL... especially every single DL that doesn't get rotated out often as they take breathers on the field.
And honestly I don't expect Ferrell to get outside the top 10 without something wrong with him.
But I think even said Kenny Clark took too many on-field breathers for me, before seeing a report that basically said he was taking (I think) over 90% of all defense snaps ... which is basically unheard of for a NT/large DT. I didn't blame him one bit after that, he was playing hard at the start of the snap.
Most other DTs I was looking at, rotated out for breathers and to stay fresh a lot more often, and some I think played extremely hard all the time, but only 40% of the snaps etc.
But there can be some hidden factor like that, where they look worse on the field because they're doing more work. And I'm pretty sure Ferrell is asked to play the 4 tech (which is harder/more physical and on certain play calls seemed to be asked to dive inside to be become an interior pass rusher (like a pass rushing DT) after the snap and an outside blitzer like Pettine's scheme. But that's more physical work than some edge rushers are used to... as most are used to staying at the edge only, and rushing the outside.