An Alfine with an 18T cog fitted is the equivalent to an 11-34T cassette in terms of spread (with a 20T it’s the equivalent of approx 12-38T spread). Most setups I’ve seen have had an 18T cog with a 34T ring, giving you a lowest ratio of 1:1 (same as middle ring to biggest cog on a derrailleur setup normally), and a tallest ratio of just over 3:1 (which you pretty much NEVER need anything taller than offroad at least).
Can you get uphills with a lowest ratio of 1:1? Depends on SO many variables. How strong you are, your fitness, are you a gear masher or a spinner, where do you live, etc etc. Was discussing the same thing today with singlespeedstu off this forum. Many people ride Singlespeeds pretty much everywhere (with approx 2:1 ratio) after all, but my legs aren’t that strong personally. I also prefer to sit and spin rather than mash. On rides with steep terrain, I use my granny a lot, but on todays ride, which was mostly rolling hills, with a few longer climbs but only 2 short/steep climbs, I barely touched the granny ring.
My thoughts are along the lines of putting an Alfine on my hardtail, and using it for most of the riding I do now, but if I know the terrain is going to be steep and there’s going to be a lot of climbing, just take my XC full sus bike with a full compliment of gears instead.
I will however be doing my damnedest to get a bit of a ride on one first before I decide it’s the route to go down or not. Having had a bit of a go on a Rohloff before, and being very unimpressed by the amount of drag in the Rohloff hub, I want to check for myself that the Alfine isn’t as draggy (though I’ve been told it isn’t).
