It's specific impulse ('fuel efficiency') is unbeatable. That buys you a lot more cargo capacity for the same sized rocket. And since this program is putting large items into lunar orbit, you need that badly.
But as you said, it's... complex to work with.
Hey it worked for the space shuttle.
Liquid hydrogen is very low density, and requires extremely low temperatures.
This means that you need a much bigger tank for a given weight of fuel, and that tank needs to be insulated bc liquid hydrogen must be kept below 33 kelvin.
Both of these downsides can come close to negating the upside of increased ISP.
But the upside for SLS is that they already had engines that can run on hydrogen. Like you said, trade offs all around
I think you mean the same mass rocket, because hydrogen is not dense, which factors into the tradeoffs (more structural mass to contain it).
Fortunately bigger tanks get better and better the bigger they are. The mass of the tanks is mostly a function of it's surface area, growing with n^2, while the fuel stored grows by n^3.
And the SLS tank is pretty darned big indeed, with a very large diameter.
Etc. etc.