ofc lo + tail glow manaphy is powerful but that doesn't displace the viability of critdra...
i talked to pk gaming a few days ago about this, and he pretty much said that when you run kingdra, it is either rain sweeper or crit. critdra has some awesome things to it. first is obviously the power behind crits. second is the thing i think you're missing spammable no drawbacks 140 bp stab. that's absolutely ridiculous, and you're crazy if you downplay the effectiveness of that.
252 SpA Sniper Kingdra Draco Meteor vs. 252 HP / 252+ SpD Deoxys-D on a critical hit: 199-235 (65.4 - 77.3%) -- guaranteed 2HKO
+3 252 SpA Life Orb Manaphy Surf vs. 252 HP / 252+ SpD Deoxys-D: 204-242 (67.1 - 79.6%) -- guaranteed 2HKO
And if you run a Modest Kingdra because you don't really need Timid:
252+ SpA Sniper Kingdra Draco Meteor vs. 252 HP / 252+ SpD Deoxys-D on a critical hit: 217-258 (71.3 - 84.8%) -- guaranteed 2HKO
And the Water STAB:
252+ SpA Sniper Kingdra Hydro Pump vs. 252 HP / 252+ SpD Deoxys-D on a critical hit: 184-217 (60.5 - 71.3%) -- guaranteed 2HKO
Not that much less powerful, and you have that excellent Draco Meteor.
I chose Deoxys-D as an entirely random target; don't read anything into it, it's just for a power demonstration.
Kingdra also has an arguably better typing that Manaphy, leaving it not weak to Grass and Electric while granting it benefits like 4x Fire and Water resist. It gets weak to Dragon and Fairy, true, but:
252+ SpA Sniper Kingdra Hydro Pump vs. 252 HP / 252+ SpD Sylveon on a critical hit: 216-256 (54.8 - 64.9%) -- guaranteed 2HKO after Leftovers recovery
So Sylveon can't even switch in after Kingdra has set up Focus Energy. And we all know that Draco Meteor wrecks Dragon-types...so nothing that it's weak to can do much besides revenge kill it (other than Azumarill, which Manaphy has a better match-up against due to Energy Ball.
When another Pokemon that's not Exploud has a no drawbacks 140 BP STAB after one turn up and can wreck some of the bulkiest Pokemon in the tier with ease, you tell me Critdra isn't good. But, based on the evidence, I can't see why it wouldn't be.