My points on each of the three:
Regice Pros:
A better Lapras, its tankiness and DPS vs double weak targets is pretty good. (Mostly in Rayquaza raids)
Potion Efficient Gyming (As long as it deals neutral or SE damage)
Cons:
Is outclassed by Articuno, not future proof at all, as Mamoswine has about the same bulk and way better DPS.
It's DPS lags behind most Ice Types.
Stuck with a one bar move in Blizzard.
Doesn't have much use outside of double weak bosses
Registeel Pros:
In the top 5 Steel attackers (Not much competition there)
Okay tankiness, but somehow less than its brothers
Cons:
Bad stats in general.
Outclassed fully by Metagross in ALL RESPECTS.
Steel type attackers typically aren't needed anywhere (Outside of Ice type bosses that don't have a Water subtype)
Regirock Pros:
Will be easier to obtain than all Rock Types, for a time.
DPS against doubly weak bosses will suffice, has more bulk than Golem and Omastar.
Most players don't have 6 Tyranitars with Rock moves
Cons:
Needs Rare Candy to fill the niche it has
Most players already have teams of Golem powered up
Community may not want to raid it, other than for dex entry
DPS is crucial, it may be too low in some cases
Tyranitar outclasses it in every respect (Except VS Bug Types, where Regirock has slightly better TDO) and it had a CD.
Those are my points. Do you agree?