They are what they are, we booked a Dreamliner from Manchester to Rhodes in 2016, lovely flight over, on the way back it was changed to a 767 which of course is not the same level in terms of comfort but was one of those things and got the money back on our pre booked seating. (Not that I personally minded as maybe fewer chances to fly 767s!)
The Eastern European operators these days are generally decent enough and Smartlynx etc have had very good relationships with the Western operators.
Expect though flights will be condensed so multiple daily flights to destinations pushes into one on a TUI AC etc as pointless if flights are not full sending 2 or 3 flights a day to Palma or Alicante for example this summer.