I'm starting to really like gas-cooled reactors like pebble-bed. They have two fundamental technical challenges. First is that gas coolant (typically helium) isn't a great heat-transfer mechanism so you need to spread your heat generation out in a large volume. This low power density results in pretty large structures (which can get pricey to build) for relatively low power, compared to other coolant configurations like water and especially liquid metal. Related, if there's a coolant leak, all the pressurized gas shoots out and you need backup cooling systems or particularly low power density that supports heat removal via conduction and thermal radiation. This can ding your economics pretty bad too.

Second, the fuel particles you mention are very expensive to fabricate, $10k/kg. Estimates of future costs go "as low as" $3k/kg, but some say it could even go up to $30k/kg. In all cases, that's really expensive fuel fabrication. There's a plant in China that can fabricate them in moderate bulk right now, so perhaps with time and experience we can figure out how to bring this process down in cost.

Aimed at solving power density and decay-heat cooling issues, there's also the salt-cooled version of the pebble bed reactor which gets the high-temperature fuel benefits and trades high-pressure gas for low-pressure salt. A research group from MIT and UC Berkeley has worked on this for years under Prof Charles Forsberg, and a company in the Bay Area called Kairos is now working on commercializing it. Unfortunately you get a lot of tritium production from the best known salt, which is a FLiBe salt, where the Lithium + neutrons results in crap-tons of supermobile tritium. You can cold-trap most of it but it's still a pain. And the fuel is still $10k/kg to fabricate as far as anyone knows, and the graphite cracks, and high temperature corrosion under irradiation is hard.

FLiBe also needs Be. World annual production of Be is 230 tonnes. And new Li isotope separation facilities would be needed, as only very small amounts of 6Li could be tolerated. The existing technology for that is not workable now due to mercury leakage.