If it is called the Steinitz defence it cant be all that bad. Steinitz himself employed it because he wanted to keep as much pawn control over the center as he could, but as others have said, the resulting positions are a bit passive and not to the taste of modern grandmasters. It is playable by unsophisticated Black players because generally they only face unsophisticated White players who do not know how to exploit their small advantage.
The one piece of theory you really ought to know is the "Tarrasch trap"
1.e4 e5 2.Nf3 Nc6 3.Bb5 d6 4.d4 Bd7 5.Nc3 Be7 6.0-0 Nf6 7.Re1 0-0? 8.Bxc6!
Bxc6 9.dxe5 dxe5 10.Qxd8 Raxd8 11.Nxe5 Bxe4 12.Nxe4 Nxe4 13.Nd3! f5 14 f3 Bc5+ 15.Nxc5 Nxc5 16.Bg5 Rd5 17.Be7 and wins because if 17. Re8/f7 then 18.c4! (Tarrasch-Marco, Dresden, 1896) This trap is theoretically important because it shows that Steinitz original plan of maintaining a pawn center cannot be achieved.
Amusingly, I once played in a tournament where 7..0-0? was played against a friend of mine. My friend was aware that this was an error that should lose at least a Pawn, but could not recall exactly how it went, just that it began by exchanging everything off. He was reluctant to do this, because if he did make all the exchanges, but still could not remember the final moves, he might only draw against a weak opponent. So after thinking for a very long time, he refrained from the exchanges but eventually lost on time.