Honestly, I would rather seven per island than seven across all four islands. Across the four, or five as it may be, islands, that allows less than two challenges per island, and for the actual size of the game, that's a pretty large underutilization of resources. Assuming that the Totem Pokémon do account for a challenge, we already know that one of the challenges is accounted for as a Gumshoos-Raticate combined challenge. The first rodent, even an Alolan Raticate or the successor to Diggersby, isn't exactly an awe inspiring challenge or game play element, unless it is just one of many.
That said, there is a third option. My impression was that the review team did not cover the whole game. If so, then isn't it possible that they only encounterd seven challenges, or even that the first island only had seven challenges, but that the number wasn't necessarily consistant across the four islands? There seemed to be a lot of ambiguity in what they actually meant, from what I read. Either way, seven to the game as a whole - less than the standard eight, isn't the kind of feeling that Sun and Moon and all of it's fancy newness has displayed, and would in the long run fail to live up to some of the hype, I think. Seven certainly wouldn't be enough if we really are expected to build a league from the ground up.
I think it would be rather interesting however, if rather than sticking to the theme of type based gyms, there were some Battle Frontier-esqe challenges involved as well. Then, as a part of the post game, these fascilities could be revisited, sort of like the Battle Frontier rose alongside the league. It would add depth to the game, the story, and the region, and lend to a strong post-game that we have a reason to feel personally invested in.