Interesting question. For what its worth this is my take on this.
So we are assuming that we have a hammered instrument that uses multiple mallets versus a keyboard instrument.
There are limitations for the hammered instrument because of the action involved. Holding two mallets in each hand is ok because you can angle the strike to use either of both of the mallets. There are people who can manage three mallets per hand and I stand in awe of them. And yes there are people who play with four mallets in each hand but it becomes harder to manage and I have always thought that the performers using three mallets per hand manage more complex music that those who use four mallets per hand. I seriously doubt that anyone can manage five mallets per hand and play anything meaningful.
So there is a major difference. On a keyboard instrument you can play from 1 to 10 different notes at a time (or sometimes 12 notes by striking more than one with a single finger) and you simply cannot achieve that with hammers by yourself. Add one or two extra players and you might have a chance but then space would be limited and they would almost certainly get in each others way.
Having said all that I did once go to a concert where there were two very talented marimba players playing two instruments and I have to say the music was, in some places, very complex. However there was no nuance to the playing: there was no tonal variation and not a lot of dynamic variation which is something you would want to be doing in a contrapuntal piece for example.
So my feeling is that the hammered instrument is hampered by the drawbacks of being too difficult to play (solo) and not having enough tonal and dynamic variation to make really complex music easy to listen too. Still love the marimba though.