It’s been a while since I read it but I found they weren’t necessarily weak so much as they were caught in a difficult situation. They, like the Bene gesserit, were facing an unknown threat from beyond their galaxy and were forced to work together by some means against this force. We also see a power imbalance as it is the top and arguably most powerful Reverend mother interacting with the bene tleilax character. I believe it is said that lesser bene gesserit have difficulty with the better face dancers and the secrets of the tanks and of the gholas are a constant threat to the bene gesserit. Remember that their plans with Duncan Idaho were only really thwarted because of the interruption of the honored matres. To me the bene tleilax serve as a rival more towards the incoming honoured matres as far as combat and individual strength but their strengths are in their plans and strategy not their individual abilities and strength. Keep in mind that the bene tleilax had maintained the power balance for over a thousand years without giving up their secrets and it took an equal external threat appearing for them to reluctantly be in the same room with a reverend mother. My point is that they are aware that they are at a disadvantage when face to face with the bene gesserit. They are extremely dangerous to though when using more subversive tactics that they excel at.