I don't think you should refrain from using an examiner just because he's tough. My private was a cake walk - 45 minute oral, 1.2 hour flight. Everyone wants to have the best pass rate possible, but if it makes your students better pilots - having to more precisely and know their material better - then I don't see what the problem is. Just next time make sure your student is 100.0%, instead of 97%.
In many ways I wish I'd had a harder private checkride, because if I'd gone out [like so many other people do] and immediately started doing a ton of solo/xc flying by myself the day after my checkride, I might have gotten myself in trouble before I learned any better. But if you go to the hardest examiner around, study appropriately, and then pass with flying colors, then you'll have better confidence, better knowledge, and better skills.
It sucks that the student busted, but personally I don't really buy into the policy of sending students to the easiest examiner you can find if it means that the students get away with inferior knowledge and skills.
There's a difference between a tough examiner, and one that wears down the student with a totally unnecessary 5 hour oral, to where the student is already behind the power curve for the flight simply due to fatigue.
On a checkride, you don't want to make it cake, but you do want to set the student up out the gate for success, not failure.