I have been having the same debate since the advent of ConnectMiles.
To your principal question of upgrades: yes, plentiful upgrades. If you achieve Presidential Platinum (100k) miles with CM as you suggest, you'll get upgraded almost every flight within latin america, with slightly lower probabilities to GRU, EZE, and SCL. I don't have much experience flying to the US on CM aside from Florida. And I don't know how the new Max9 planes will affect upgrades.
I tried ConnectMiles for a couple of years but now attribute all my CM flights to MileagePlus because I need them to get premier status on UA. It works out much better in my case because most of my flying to the US requires a domestic connection through IAH and a separate upgrade opportunity. If my "base" in the US were one of CM's direct flights, I might change my opinion. I don't have any complaints about ConnectMiles.