A revenge drama is a tricky business. You don't want it to be a too much of exploitation trash, but you don't want it to be too safe either. 'Femme' falls closer to 'playing it too safe' team.
I found the biggest issue was that the key moments didn't ring true. The assault scene, for example, is the basis of the rest of the ensuing drama. I wouldn't have expected it to be the level of 'Irreversible'... but it definitely relied too much on camera works and editing than the rawness of the violence, thus making Jules' resulting trauma less gripping to the audience.
The following key moments of shift also failed to be convincing. How Jules re-encounters Preston, how that chance encounter leads to hook up, how that again leads to a proper steak date... all seem to just happen for the convenience of the plot. Also Preston's changes as a character, namely how he develops deeper feeling towards Jules and admits his submissive side... all come along without much development (after literally one scene after another of him using Jules like a sex toy, that is).
Together with Jules' frankly flimsy plan of 'revenge' (why would he bring Preston back to his place and let his friends meet him if he truly was serious about his revenge?), the overall result is that it made it hard to take the whole thing rather seriously. A revenge drama that keeps you at the edge of the seat, 'Femme' is not.
One of the weaknesses was also Stewart-Jarrett playing Jules. His switch between feminine-masculine persona was impressive, but his display of tension/fear/emotional struggle was pretty mono-toned. Say, his fear before the assault should have a different colour to his fear when he later faces Preston's thug friends, for example. But to my eyes, it all felt pretty generic 'fear'.
Mackay's display of explosive temper and the contrasting soft/weak side is great, but even his great acting cannot quite make up (already mentioned) lack of convincing development.
I want to see more risk taking and raw approach when the film maker took up such a controversial subject to start with.