mobile

Hello, i didn't find any answer so here's my question :

ROM :
What's the differences between 2 customs ROM ?
I mean, i have seen like 10 different ROM based on the same android version but i don't see any difference.
Why shouldn't I stay on Cyanogen ? I don't get it.

Kernel :

I think i kind of understand the benefit on a custom kernel (overclock, maybe better battery life ?) but isn't it too risky ?
How does kernel compatibility work ? Is it based on an Android version ? Do 5.0.0 kernel will be compatible with a 5.1.1 version ?
Do you really feel the difference compared to the stock cyanogen one ?

And my last question will be : does cyanogen 12 (and 12.1) have OTA update or do i have to flash new zip every day ?

Thanks !!