As with any engine there are good points and bad points. The RR 250, when properly maintained and flown is a reliable piece.
The good points are that it is well developed and runs just fine.
The bad points are that it has a lot of piece parts and these can wear and cause problems. You need to make sure the engine is overhauled properly and not just pulled apart and the apparent bad parts replaced. The turbine disks have a finite life and you have to respect those life limits lest you burst one and ruin your day.
The engine is old technology and sucks a lot of fuel for the power you get out of it.
The RR300 has a new compressor, but it has a lower flow and pressure ratio so even though the comrpessor efficiency is improved the SFC as % of power isn't better than the older engines. At the power point used in the R66 it is probably better than a 250 would be at that power, but it still sucks a lot of fuel for what you get out of it.
I don't know if they are still using the single channel FADECS. When those were working they were great, but the "reversionary mode" on the single channel system was dangerous.