IMHO i don’t find they work, it’s just a confidence booster i suppose. I’m pretty much certain that you won’t get more bites on a flavoured one than a standard expander, they are so good anyway that they don’t need flavourings. Just my opinion tho.
Some venues have certain flavours that work well but again i find it’s just the bait your using that works well and not the flavour.
Realistically tho i suppose the flavour is so it leaks into the water to get the attention of fish that are far away. Who knows what fish think, it’s just your preference i guess.
One venue i know reacts extremely well with Richworth’s KG1 pellet and so the fish but like them because of the flavour. I think in the summer you wouldn’t be able to tell if one bait works betetr than another with a flavouring on because fish will eat anything possible in the summer. However in the winter when a change bait works well (sometimes) then this will eb the time to try it when fish are quite fussy.
It’s hard to give a definate answer ~think