Underfoot zones only issue is the amount of hit points they have, and damage they deal out, however they are going to be much more deserted than House of Thule.
Going to Lichen Creep would be comparable with going to Morrell's Castle, because of how much hit points the monsters have, they also hit a good bit too as stated above.
I would just get a bunch of melee dps, berserker or monk being the best two in my opinion, and dropping them in zones like Fort Mech, House of Thule, Feerrott, even Plane of Fire, Hate, Dragonscale Hills, I mean really anywhere you want that drops Diamonds or Blue Diamonds.
I do agree, Underfoot is probably more profitable than the majority of House of Thule zones if you can kill and pull quick enough, the bone crafters in The Grounds drop diamonds left and right.