So, you roll 2 dice and if the result is the exact average, that's a critical success and if it's an extreme, you fail. That way, bigger dice have bigger chances of critical hits and lower chances of failure. I like that.

What I don't understand is the "roll one step lower" (i.e: 2d10 becomes 2d8). Is that supposed to be an advantage or not? Because on a critical hit, your skills go down (and you become worse? ) and the archetypes roll lower in certain situations (so they are worse on those? ). I think I am missing something here because on a lower dice, rolls are worse. 

Having the smaller dice range is actually an improvement, but it's definitely non-intuitive.  As you decrease dice size you get a much higher chance of a critical success, even though the critical failure rate goes up, so it net works in your favor.

Here's a link to an any dice to help visualize it: https://anydice.com/program/1049f