Bajdi:
I've run ATmega328's @ 16MHz @ 3.3V. One is connected to a nRF24L01 wireless module and a couple of I2C sensors. Have not had any issues.
So have you tested this over the full temperature range, over the full voltage tolerance range. Have you tested the voltage thresholds of the inputs, the voltage outputs and loading. The propagation delays and all the machine op codes? How many samples have you tested?
No I thought not.
A simple functional test tells you absolutely nothing but yet beginners often think it is the be all and end all. It is not.