I think the idea behind the comparison between Manaphy and Garchomp is that their approach to people trying to wall them is the same - stat up and blast them with something neutral.
It does seem to suffer from four-move-slot syndrome, however, from the discussions. No Signal Beam means Abomnasnow/Vaporeon/Ludicolo wins, but no Ice Beam means lots of random stuff wins. Flygon and Dragonite, for example. Not sure what lacking Surf would bring out of the woodwork - NP Infernape, perhaps.
Plus, if you want to abuse its ability, you need rain support. That costs a moveslot on someone, quite possibly Manaphy. If you want to really abuse it, you lose another Manaphy slot. It's really only got 3 slots already, with Tail Glow. Hydrorest brings that down to 2 or even 1. How effective can that really be?
We may find that in practice this makes Manaphy vulnerable - it could be like Lucario. Some of the movesets we're suggesting are practically special SDLuke, anyway. :P
I do wonder how effect a no-Tail-Glow set might be. Say:
Manapy@Specs
Surf
Ice Beam
Signal Beam
HP Fire