My understanding is that what makes eVent special is that it is waterproof, but more importantly, has good breathability within a certain temperature range.
If memory serves me correctly, freezing temperatures are an issue for eVent.
Again, this is probably all relative....maybe a better statement is that eVent has a better combination of being waterproof and breathable vs. GoreTex?
I have a pair of the 1st generation 2 ply eVent MLD Rain Mitts, which I have used in the rain while using hiking poles.
I can tell you that the pressure/force of my hands gripping my poles forced water through my mitts and my hands were soaked. (Wearing latex gloves inside the mitts really worked well for me).
However, I have since purchased the newer 3 ply eVent that uses different materials for the palm. (Have not had a chance to seam seal them and use them in the rain).
That is the only experience that I have had with eVent "failing", but my usage of the poles makes for an unrealistic expectation for the eVent to work.
If I did not have hiking poles, it is likely that my hands would have remained dry.
As for Cuben, I don't have any experience with Cuben in the rain.
I recently purchased the MLD Cuben Solo/Mini tarp for my setup, but rarely have encountered rain in the Sierras, so I have not had a chance to see its performance in the rain.
How do you like the Cuben floor in your bivy?
I have the old green silnylon 2.0 fabric, which MLD no longer uses, but mine is bomber strong and has never wetted out from use in the rain. (13 hrs of rain at a base camp just below the top of Mt. Whitney with MLD Ponch Tarp and MLD Bivy with crazy strong wind that destroyed other traditional tents. Sold on build quality of MLD).