Piera actually sets repeat crops - 3 different bloom bud sets and upto 13 different flushes.
The others do not do that. No loquat including Shambala sets buds outside of late summer.
Piera is definitely unique and one to get if you are in a marginal climate for loquats (zone 8 or 9a)!
Also since it is a bud sport of Algerie, it is a high quality loquat, not a weird variety that has nothing else going for it.