I was on TUN-CDG recently, operated by Amelia. I had booked a business class flight with miles, and had later received the "you've been downgraded to economy" email. Overall the experience was totally fine and roughly comparable to an AF-operated flight apart from the catering.

• There is priority check-in and boarding at TUN. The priority boarding was enforced well, with several passengers told to join the other queue. My boarding pass said Y on it, but I have platinum status; I'm not sure what might have happened if I had no status.
• The first four rows of the aircraft all had the middle seat kept free. There were those plastic dividers between rows 4 and 5 together with a curtain, but the curtain remained open throughout the flight.
• Sandwich, biscuit and drinks (wine, beer, juices, waters, cokes etc).
• Seat pitch was noticeably less than AF, and it would have felt cramped without the middle seat free.
• Cabin crew were nice.
• Parked at a remote stand at CDG and bused to the terminal.

The lounge at TUN has some new chairs but it's still bad. I ended up sitting outside to get some peace & quiet and a working wi-fi signal, neither of which were available in the lounge.