In the first Deathwatch game I ran, the Librarian quickly became the most powerful thing in the game by a long shot. With careful buys of anti peril talents and upping psi rating, a Librarian can do 10D10 blast 10 attacks every round then jump pack into combat and do large amounts of melee damage with a force sword (well beyond what any other melee character could hope to do), and that wasn't even considering the other powers available which allows them to also be very good in an anti tank and anti horde roll (depending on chapter powers available admittedly). The problem wasn't that the Librarian was too strong (just add more and harder enemies) it was that the rest of the players became pointless, their just to keep the enemy busy until the Librarian arrived to save the day.
Now, I know that there are always enemies and settings that can be used to combat this, but it ultimately came down to two things. Either I crafted every fight to make it difficult for the Librarian but not the rest of the team (which would have stretched believability) or just to ban Librarians, which is what I did, starting a new game without them.
A little while on from this, I've decided to try and re-introduce Librarians into the game, but obviously the same problems exist. What I'd like to ask of the players here is have they come across the same problems with Librarians in their games? If so what did they do about it and if not, what was stopping the players from taking the overpowered route?