I came to my 2018 Offroad Premium from a 2013 H6 5 EAT Outback that saw more 4x4 trails than most 4Rs will ever taste.
There are huge differences, obviously, even though there is some overlap as well.
Offroad:
The Outback with good AT tires will get you places most people think it cannot, especially if you get the H6 or coming turbo version. However, it is nowhere in the same league as an Offroad Edition 4R. I am not only talking terrain too hard for an OB. I am also talking terrain like the Alpine Loop or the White Rim Road where the 4R will be faster and more comfortable. If you think the 4R is not comfy offroad, wait till you try a crossover.... Another disadvantage of a lesser vehicle offroad is the inability to easily use 4x4 guides and to explore blindly. I always learned as much as possible about any 4x4 trails I took the Outback on. You cannot easily figure it out on the basis of guides like Wells' for UT/CO/AZ/CA. But if you drive nothing worse than rough forest roads and easy 4x4 trails then the Outback can compete, definitely better handling and speeds on 2wd terrain unless you upgrade the 4R suspension to a performance one. On moderate trails, a stock 4R will obliterate the best prepared Outback (mine was pretty well prepared). You are not taking an Outback on any difficult rated trails (unless they have been improved recently, like me on Governor Basin).
On road:
Handling and cornering: I have not installed my 6112/Icon 2.0 combo yet, but the 4R Offroad Edition on stock suspension--or stock shocks with aftermarket coils (as I am temporarily)--is a joke compared to an Outback, even in dry weather. So this is the flip side of the off-road situation.
Maintenance/reliability:
I got my 2013 H6 5EAT for being about as reliable as non-Toyota vehicles get. With nearly 4,000 unpaved miles I was on stock shocks at 73,000 when I traded it in. The CVT reliability is one of those debates that won't end soon. The high performance version on the H6 and XT models performs great on road.
Cargo: The 4R kills the Outback. It is not even funny. This was even more important to me than the offroad capability. In addition, the 4R is also much better than the Ascent despite the nominally equally sized cargo areas. The 4R's is much better shaped for cargo whereas the Ascent is really made for that third row of seats with a cargo area that is too deep and not tall enough; it also sits much higher than the 4R's.
In sum, there is overlap, but they are truly as different as their constructions suggest. The 4R rocks offroad and in hauling stuff, the Outback is a joy to drive in rain and snow. Bear in mind that in places like AZ, the OB sloping windshield exposes you to the sun a lot more than the 4R's small, more forward located windshield.
EDIT: I should add that I am not a fan of H4 CVT (or previously 4EAT) models. They are too basic and utilitarian for my taste and excel at nothing. They are really for people who consider their vehicles nothing more than an unfortunate necessity. When loaded up heavily, they are not very economical either.