yeah I think Phenolic roughly translates "hard crumbly cheese" 🙂
i've just had the last uplift weekend at Fort William ruined because of these sh!tey Phenolic pistons in my juicy 7. Basically the central locator snapped off and the pads kept dropping through the brake. Granted it did snap off because of Screwdriver use to push pads apart (incidentally why has no-one made a tool for this) but I thought the "pad retainer" might have saved the day. Well, it cost £2.99 and does **** all – didn't hold the pads in either.
The pistons themselves are cheap (well they used to be about 4.99) and you can break the caliper and do it yourself – takes a few hours and a bit of patience to bleed but results are good. TBH if an LBS charged me £70 for a brake service i'd just buy a new one – Shimano SLX for £60 from CRC right now.
If the piston is chipped it depends how bad it is – mine no longer have a smooth edge but still work ok.