Hydreigon is bulkier for OU standards Cherub where stuff hit way harder. Nidoqueen is bulkier relatively to Hydreigon in OU. Hydreigon also has weakness to common priority, which means that even common slower Pokemon can deal with, it can be abused as it has to lower its SpA to fire off its best move, is much is easier to pivot around in general, loses life when attacking, and is prone to Toxic, Burn, and paralyze (and Roost only helps against burn really, and not that much, as taking 22.5% each turn is a lot anyway).
Part of the OU QC team, message
me for a check!