We have a pellet boiler, but since a pellet blowing truck can't get very near the house, we have to buy the pellets in 10kg or 20kg sacks (definitely not a good option for anybody in their eighties!). Apart from once when we had blockages with 'economy' pellets which were very dusty, the boiler has been problem free so far. It is also very economical to run and creates next to no ash (i.e. one bucket full every 6 months).
The one drawback for us is the space taken up by the boiler, thermal store (a big tank of hot water) and pellet storage. Our setup takes up about the same room as a car.
THe RHI scheme is great, and pays for a big chunk of the capital. However, beware of the salesman's calculations on how much money you'll save. Each of the companies we asked for quotations gave us very misleading calculations in this regard (easy for an engineer to spot, but less so for others not in the know?). Also, the calculations they do to say how much fuel you'll get through were very wide of the mark, probably in an effort to magnify the predicted savings and also maximise the RHI benefit that could be claimed(?). In practice, we use perhaps a third of what the calculations predicted, so goodness knows what temperature you're supposed to keep the house at!?
For your parents, the absolute worst case would be to assume that a new boiler would have the same efficiency as the old one. Then you could work out their predicted consumption just by adjusting for the % difference in calorific values between coal and pellets and at least that will give you a pesimistic estimate rather than optimistic.
The only people I know who have had problems with pellet systems are either people who bought the cheapest possible boilers, or who have very complicated installations blowing pellets over long distances etc. However, if you buy a good quality boiler (e.g. German or Austrian) and keep the installation as simple as possible, I don't think you'll regret it

.
I hope that helps!