Just looking at the central heating boiler systems, when you consider:
- bulk storage of pellets can be an issue
- getting an local experienced technician to service them can be an issue
- there is a lot more general maintenance required by the home owner
- pellet quality appears to vary a lot and can be an issue especially for automatically fed boilers
- regular parts replacement required (e.g. ignition elements)
- many of the original installer companies went wallop in the 2008 crash leaving many homeowners high and dry
- pellet price movements seems to mimic oil price fluctuations
I would question whether they were ever a viable alternative to oil or gas boilers.