The Alola League is a legitimate Pokemon league. I think it provides just as much challenge as other leagues, if not more!
In Tetris 99 and Fortnite, it can be tough to be in the top 16 of a match, so imagine that in the Pokemon world. You have to survive 151 trainers to be in the top 16! That's tough!
Then the top 16 and top 8 matches are one-vs-one. If you choose the wrong type of Pokemon, you could be messed up right at the start. You have to know your opponent and your Pokemon to survive these rounds!
Only the best of the best can make it to the top 4!
I do think that's what people overlook.
If you put constraints on who joins and who doesn't, you tend to push out certain people who could easily win the entire league. Maybe they don't have the luxury to get out there and collect all 8 badges, for a variety of reasons. If a League is open to everyone then you get a situation in which a lot of people have access to. Sure there will be weak trainers but there will also be strong trainers.
That was even the case for past generations where some of the trainers in the league, made you ask "Did this person really earn 8 badges? Were they pity badges? Because I can't see how they managed to get 8 badges, given how easily they were defeated in comparison to how hard it took Ash to get those badges but he's obliterating some of these trainers.
Though I will agree, strict constraints do make sure that a good majority of strong trainers will show up, but I do think it does push some really strong trainers out of a tournament for various reasons (like time only enough to enter a tournament not time enough to gather all the badges).
And as I've mentioned before, they really couldn't do much for the Alola league since it was the first league. How can you add constraints to a league that you suddenly JUST announce will happen in a short period of time. There wouldn't be enough time for there to be enough trainers, better to start of lax and then work towards perhaps more limitations.