Secondly, the broad set of hidden information of the tiles makes Mahjong a much more difficult imperfect-information game than previously studied ones, such as Texas hold'em poker. Thirdly, the playing rule of Mahjong is much more complicated because of the various actions involved.
It's primarily a skill game, although luck plays a huge role. In any single given hand, luck is the predominant factor in who will win or lose. However, poker players don't play a single hand. They play hand after hand after hand & and overall, skill dominates.