I'd say that on a pure land map Amphs only got 2 good units. Buoy and Grizzly.
Buoy is a great deathballing tanky skirmisher. If you avoid losses and repair them there are few other similar priced units that can approach them.
Grizzly is a great heavy skirmisher. It pretty fast and if you can prevent it against spam and anti-heavies it's in its' legue of its' own killing Dantes, HLTs, Sumos and even Goliaths.
Ducks are basically the worst raider on land as it cannot hit dodging raiders, is slow and expensive.
I can see use for Archers in gimmicky ways like pushing enemies into defense or pushing units off cliffs but I am rarely able to use them for that.
They can protect Buoys against enemies but if they just keep pushing enemy units away the Buoys will miss and the enemy will be able to escape and repair.
Scallop is slow and have low range so in order to be effective the enemy basically have to run into them.
That said.
When the Amphs can use water they gain free repairs in the water, they can hide and escape from enemies and ambush enemies from there.
I'd say that Amphs are most effective when they can use their Amphibious abilities to escape from ships and land units and ambush enemies.