The problem is not with individual premium packs, it is with overall premium squad composition.
They become less powerful than regular squads pretty early in the game, let us say somewhere in first quarter of campaign time-wise. You won’t spend slot on them after level 12, or earlier.
Most of players won’t spend money on toys that are breaking that quick. It means, as soon as people figure it out, the packs won’t sell, which will lead to less varied game.
An ability to train regular soldiers would create a reason to keep these squads even after level 26 without breaking the balance.