Yes, but it's bulky which means it's more likely to get stuck when pulling rappel ropes and takes a lot of time to tie. Use an overhand instead. For joining cord, I use a fisherman's. How often do you need to untie those knots? I hardly ever do.
Note that in a proper Flemish Bend, the tails come out of the knot on OPPOSITE sides. If you tie the knot such that the ends come out on the same side, the knot will roll under load and you will die. I think that is what your well-meaning critic was thinking of.