Hi Keith, and thanks,
Can't say I've had a duff C821 myself, just a suggestion from a Quuq hero who seems to have some considerable experience of faulty 25-4 boards, and feels that the cap can be/is suspect with an PFC/Inverter supply Pop.
As to caps generally, tend to use whatever is cheapish first, then if any problem, quickly source something better. Wiltsgove low ESR caps pretty good - never yet a failure.
Customers naturally get peeved with premature failure, so try keep on the ball - Chris.