When I got my Nightfox a few months ago, I decided I’d just go with the 138mm Venture II cranks it came with. I think that was a prudent decision that allowed me to get comfortable freemounting and riding. The first 36er I test rode had 125mm cranks, and I wasn’t able to freemount it, but I was later able to freemount my Nightfox with 138 cranks on the second attempt. Once I got used to the 138s, I wanted to try out some 125s, but I wasn’t sure if I wanted to buy a new set and find out I didn’t like them.
As luck would have it, my wife just bought me a Nimbus Signature trials that came with 125mm Venture II cranks, and some of my trails buddies suggested that longer cranks would be better for jumping and trails stuff, something like 137/138mm. Well, that was a no brainer, and after borrowing a crank puller from one of our local unicycle club members, I swapped out the cranks between the Nightfox and the trials, and I think it was good choice for both.
I took the Nightfox for a spin today, and it was pretty cool. Even though it was a bit harder to freemount, I was able get it 50% of the time today, and I suspect that will get better as I get more familiar with it.
The ride was sooooo much better with the 125s! It felt more natural, and my cadence was noticeably better and more even. I also just naturally rode faster. I actually like the resistance the shorter cranks provided and reminded me more of what it’s like to ride a geared bicycle in a higher gear. Also, less leg movement caused less bouncing around, less crotch pain, and a smoother overall ride. I do feel a little less in control, but I felt even worse when I first got the Nightfox with the 138s, and I expect with repeated riding I’ll have more control. I didn’t have a single UPD today during my 8 mile ride, but there were a couple of times I felt close. I also haven’t ridden my Nightfox much lately, and I’m a little out of practice, so that probably played into it. I have, admittedly, been fairly distracted with the trials unicycle and learning tricks, hopping up stairs, etc. 
Uphills were not as hard as I though they would be, although coming back I ran into a nasty headwind, and half of the way back is uphill, so it was a bit tough getting home. I really didn’t enjoy fighting that wind, especially while trying to get used to the shorter cranks.
The other thing I should have spent more time considering was the saddle height. With shorter cranks, I probably should have raised the height with the shorter cranks. My knees were a bit sore, so I’ll play with that a little later on and try to find out what works best. I think subconsciously I sort of wanted to keep it a bit low for this first spin since I was intimidated by the shorter cranks and wanted to make it easier to control, but now that I’ve tried 'em out, I’m more comfortable.
As an aside, the longer cranks worked out well on the trials unicycle. I had to lower the seat, which required cutting an inch or so off the seat post, but I could still do all the tricks I just learned. It did feel a little odd, though. I’ve never used cranks that long on a 20" unicycle, but it does seem more stable.
I’m hoping to upgrade to an external d’brake on the Nightfox. I was already intimidated enough with going down steep (or even slightly steep) hills using 138 cranks, and it’s even worse on the 125s. I’d also like to be able to slow down a little faster. With the 125s, it takes me a long time to stop, and while I know that an instant stop would cause a UPD, I would still like to decrease that stopping distance. My best chance for riding the 36er on a regular bases is the multi-use trail that runs close to my house, and there’s just times when I’ll need to be able to decelerate faster, and it’s not just a safety issue for me. There’s small children and pets on those trails, and I don’t want anyone or any animals getting hurt. It would also just make me feel more comfortable. I know it’s hard to compare, but I love having the brake on my muni. It helps me to ride a lot more confidently knowing that I can slow down faster and with more control when I need to, especially on steep declines.
If I go that route, I’ll probably use the 127/150mm KH ISIS Spirit DH Cranks. I’ve also been curious about taking the 36er off road, and the option of extending the cranks to 150mm seems pretty cool.
Plus, 165s? That’s almost a bicycle length crank. (I think 170s are common on mtbs.) You must have had incredible leverage. I can imagine that would have made you feel more confident. I like the way you worked up (or down) to the shorter cranks, and kudos to you for taking your 36er on trails! I still don’t even like to go fast on my 36er, and the idea of riding off road scares the bejesus outta me 
, but I still want to do it!