One thing I have noticed watching well-coached OLs, even those with replacements, is that occasionally you will see a goal-line TD where the RB went untouched. But I have seen the 49ers do it a lot, same with the Rams.
Almost every run we have had in that situation has been challenged.
That means we don't have a great interior OL. Biadasz is average, we can see that Martin is really a shadow of his former self and Tyler Smith will eventually have to go to tackle.
I cannot make up my mind if it is scheme or age or both.