The Felguard-using demonlock has trouble against locks because all they have to do is banish his pet, and he's gimped.
There are things he can do if his Felg is banished, and it does help that banish's duration is shorter.
In this fight a lot depends on - well, a highly skilled lock of any flavor is going to beat a not so highly skilled lock of any other random flavor. Gear matters, too.
The conventional wisdom on Felguards is that they are good for leveling up from the mid 50's until the high 60s or 70, when you should dump the build for a "grown up" build - I don't ascribe to that mindset, but that's the CW.
Most people would probably go against the Felguard, because most people just go against the Felguard. They do so because unskilled and immature locks are believed (perhaps rightly so) to graviated toward the Felguard - and play him rather cluelessly.
If you like SL/SL, stay with it. Felg requires maintenance - requires healing - you have to "get" how to use the Felg to be effective with him - and you have to "like" taking care of this demon to get good use out of him. He's not for everyone (and his constant mocking vocals are teeeeeeedious).