I just tried creating a Sphinx DB on a Firebird 3 database and the following exception was raised:
Project SphinxVCL.exe raised exception class EIBNativeException with message '[FireDAC][Phys][FB]unsuccessful metadata update
key size exceeds implementation restriction for index "PK_SX_STORAGE_ITEMS"'.
Seems strange as the key size in Entities is only 255, I'm using a page size of 4096, so max key should be 1024. Maybe because I am using UTF-8?