What you've said here is straight up false. In terms of raw wallbreaking power, Cube is second to none in this tier. The amount of things it blasts far exceeds the ability Landorus has when confined for 4 moveslots.
The big advantage Lando has over the other big wallbreakers are speed, typing, and movepool. It has the capability to break way more threats than Zard Y and Cube, but not in a single moveset. The other 2 have pretty limited options, but those options are all they need, as those options smash through most of the game anyways. Preparing for those threats on balance is way more intuitive since they have fixed movepools, but in no way a smaller teambuilding constraint because of how damn effective they are. Lando-I's power is in mixing it up, and if you're packing the check that beats that particular Lando-I's movepool, the thing is way less scary than Cube and Zard-Y.
Checking 10-moveslot Lando-I might be harder than checking Cube or Zard-Y for balance, but not all of Lando-I's 4-slot sets are going to be able to balance break like they can. You might not be able to build around every set, but you can build around a good chunk of them and reduce the statistical threat of Lando by a large margin.
