All of the larger portable machines can be a bit difficult to take upstairs, particularly if the stair has a winder. You need to be careful not to catch the dump valve on the wall as you go round the corner.
Having said that, I have been using Ninjas for many years and have no particular problems taking them upstairs when necessary, and I am not a young man !
I guess there is an acquired knack to it, as in 20 years I have never overbalanced on the stairs !
Bryan