The cut of the sleeves/shoulders is very hard to get right and the fact that you are sealed into them mean that cut and breathability of fabric is key. Gilets on the other hand are naturally ventilated (they have big bloody holes for your arms) so as long as the fabric is shower/windproof then you're fine. To be honest, you could just cut holes in a slim-fitting bin bag and get a similar level of performance.
Jackets are very different.
The cut of the sleeves/shoulders is very hard to get right and the fact that you are sealed into them mean that cut and breathability of fabric is key. Gilets on the other hand are naturally ventilated (they have big bloody holes for your arms) so as long as the fabric is shower/windproof then you're fine. To be honest, you could just cut holes in a slim-fitting bin bag and get a similar level of performance.