I travel a fair amount for work and sometimes take my camera gear.
We tend to fly Ryanair and Easyjet due to cost and flight times.
Having done 31 trips in the last 12 months, I can tell you from experience that its hit and miss as to whether either airline enforces the 'rules' stated on their respective websites.
If you take a bag with wheels then you are more likely to be asked for it to be put in the hold on a busy flight, Ryanair and Easyjet both seem to do this.
Every single time I have gone on with a backpack, nobody has batted an eyelid.
I have seen huge backpacks, far bigger than the hand luggage allowance which were allowed through.
I have then seen relatively small bags with wheels which had to go into the hold.
There are rules but they are not often enforced unless the flight is really busy or the person on the desk is having a jobs worth day.
I have taken a Tamrac backpack containing a Camera, three lenses etc AND a tripod attached to the back and the steward on Ryanair even helped me get it into the overhead bin

On another day, I may not have been so lucky.
Only once have I been asked to put my bag in the hold and that was because I was in an exit row seat and there was no more space in the lockers, although they didn't charge me.
FYI - Never once have I had my hand luggage weighed with Ryanair or Easyjet.
Thanks