Keeping a hand with this as your first or second land drop, hoping to see one in the top 5, is almost always worse than just mulling. The times you use it turn 0 and wiff on a land you wanted, it will cost you games.
Keeping a bad hand has nothing to do the strength of a card, I'm unsure why that argument always comes up.
This being worse than a mulligan sorta depends on how much you have already mulliganed. Digging 5, even if you don't see a land, gets you 5 closer to a land you need. I think If ii have already mulled 2 times there are many instances where this will be better (contextual to the hand you keep) and once you have hit the 3rd mulligan you have to start thinking long and hard about the risks of mulling again at -1 vs keeping this and playing the odds.
I don't think you can judge this card by that metric though because most of the play this card get is going to be in decks that already do things that a normal deck would not do like Mulling for 1 card and not playing mana. The fact that this pitched to Force of Vigor is huge and if it was blue we would not even be having this conversation. Pitch cards as well as brainstorm, Jace, and Bazzar have made it so that the low end of a card is not nearly as relevant as what the top end of it can do.