Give reasons why the y/y connection of a three phase transformer is seldom used with ungrounded neutrals?

The zero sequence impedance of an ungrounded wye is very high - approaching infinity. As a consequence, the open-circuit voltage at the ungrounded neutral of a wye-wye transformer is not constrained - and if the neutral voltage can drift, so can the line voltage. The result is that the line-to-ground (line-to-earth) voltage may be significantly higher than the system insulation withstand capability, and the resulting breakdown can be catastrophic.