Not only could "all lands" decks like zombie hunt be too good, so too could "all spells" decks.

Keep in mind that back in the day when the all/no land mulligan was a thing, such decks weren't even a concept (aside from playing casual "no-restrictions" decks with a pile of Moxen). There was nothing to be gained from running all-land decks and no-land decks wouldn't even be remotely viable. By the time decks started coming around that could have been built to exploit that mulligan (Academy comes to mind) the Paris mulligan was already in place.

So the major con that everyone cites wasn't really a factor back when that mulligan style existed. Even the 9-land Stompy decks were a couple years away.

By far the biggest actual con to the mulligan was the lack of control; a terrible hand with 6 lands and a Disenchant was a keeper because there weren't 7 lands. A hand with a Maze of Ith, a Kjeldoran Outpost, and some spells was also a keeper, despite having no way to actually generate mana.

For a while when the Paris mulligan started showing up (but wasn't official) people tried out different things, and it didn't take long to figure out how much better Paris was. For every situation in which you draw 0 or 7 lands, there are far more situations where you draw an incredibly shitty 1, 5, or 6-land hand.

So the biggest con of the original mulligan system is simply that it's not as good. You're far more often to end up with bad but un-mulliganable hands with it than with Paris or Vancouver mulligan. That's the con. For "normal" decks the only pro is that you occasionally get to mulligan to 7 instead of 6, but that's heavily outweighed by the con.