Its quite obvious that the Mangudai is a unit which could be extremly broken if buffed too much. Also I dont think adding a new effect will be something realistic in the near future. But I have to agree that in the current state Mangudai are just not good. Yeah you can try to raid with them but for the price of them (which is really high) its not a good raiding unit. A few archers, a tower or walls shut them down completly and since they need micro to kite its an even worse raiding unit.
The RUS horse archer is better in every way at the moment since they are cheaper, do more damage, have more range and get better upgrades. The shooting while moving of the Mangudai is just not as good since they have such a low range.
I made an in-depth analysis of the Mangudai here in case you are interested:
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