I have owned a UM36 for over 4.5 years and recently got a 2nd hand UM36X, both bikes are virtually the same, just a few upgrades from the UM36 to the UM36X; springy seatpost, better suspension forks, integrated wheel lock with chain, pannier rack mount points, matt black paint job instead of the strange silver/green the old range had.
The only difference I can see between the UM36X and the UM33X is the frame size, the UM33X having a larger, 19" frame whilst I've had to buy really long seatposts for my two bikes as I'm 6'2" and the orignal supplied seatposts made me feel like I was riding a bmx.
They're a nice ride, feels like a mountain bike with a bit of a kick to it because of the motor, the only main things that have given me trouble are the things that even ordinary bikes have trouble with, brakes/tyres/chain/sprocket all wearing out through use, the motor & controller of my 4.5 year old (and very used) bike still work like new, batteries tend to last about 2 years with me but I gather that's about normal for most ebikes.
I was recently quoted £329 for a replacement 26v li-ion battery pack from Urban Mover, I intend to re-cell my oldest battery soon for about 1/6 to 1/4 that.
I upgraded the sprocket on my UM36 to a megarange one with a much lower first gear and that has really helped on the hills where I live, I'll be doing the same to my UM36X at some point too.
They've been making this design of bikes for over 5 years meaning spare parts have been around for that long, which I would take to mean they've invested a lot into this particular design and the parts will be around for quite a few years to come.
I do live about 15 miles away from their main buisness residence which has been helpful in the past.