Things have been busy around here, but I want to start getting some of the technical stuff from the Yuba build posted. Answering a comment, the rear hub is a Halo Spin Doctor Pro DH. Halo is a UK firm that designs relatively affordable freeride/dirt-jumping parts. The SD Pro rear hub is a cartridge-bearing design with interchangeable freehubs and a few different axle configurations. The DH version comes with the chromoly freehub and 10mm hollow chromoly bolt-on axle installed in a hubshell drilled for 48 spokes. Parts are available to adapt the 10mm bolt-on axle to 14mm, which is the only reason I went with this hub. I wanted a freehub so that I could run a modern 9-speed 11-34T cassette with a single ring up front to get the widest gear range with minimal parts.
I love the Mundo frame for it's strength, but its targeted selling price appears to have forced some design decisions I would've avoided. The Yuba has one of the most ridiculous dropout configurations I've ever seen: 130mm x 14mm. The 130mm spacing is the width of a modern road bike, but the 14mm axle diameter is only used in heavy-duty BMX parts. The stock wheel that Mundos come with has a BMX hub with a longer 14mm axle installed. They can do this beacause the hub has simple cup-and-cone bearings, and the axle is just a piece of threaded rod. The rear cogs on the Mundo 6- and 18-speed builds is a freewheel cluster, the only way you can get multiple gears onto a BMX hub that was designed to carry a single-cog freewheel.
Freewheel clusters have two major design problems: first, the right-side hub bearing is located under the right-side spoke flange, which is the entire width of the cluster away from the right-side dropout. The load of the bike is transferred to the axle at the dropout, and is transferred through the bearings to the hubshell. The length of axle spanning the distance from the dropout to the bearing is a mechanical beam, and the longer that beam is the stronger it needs to be to not bend under load. This is why the Mundo hub has a 14mm axle: the distance between the right-side bearing and dropout is large, and a 10mm axle would bend under the Mundo's rated load. The freehub design, where the freewheeling ratchet mechanism is part of the hub rather than the gear cluster, puts a bearing much closer to the right-side dropout, so a smaller axle can be used because it doesn't have to support the load over such a large distance.
This brings us to the second problem with freewheel clusters: due to the basic design reality described above, every quality hub intended for multi-gear usage from the last two decades or more uses a freehub. All the advances in shifting technology in that time frame and all decent parts currently in production were designed to work with a freehub cassette and quality freewheel clusters aren't made. So, if you want the shifting on your Mundo to work as well as on your "good" bike, a freehub is first on the list of replacement parts.
A Long Walk to Green solved the problem by brazing 4mm spacers into the dropouts to run a normal 10mm-axled hub (he used a Nuvinci CVT). The 130mm spacing isn't a barrier to fitting 135mm mountain bike hubs in a modified frame like that because the Mundo is made out of heavy steel, and you can just spread the stays apart a bit to fit the hub in there. It makes changing a tube a bit more arduous, but it's not a big deal. So, that's one way to go: modify the frame and a run a strong, standard hub.
I went another route and found the only freehub made that is intended for use with 14mm dropouts. The only problem with that is that the USA distributor for Halo, BTI, has the SKUs messed up for the Halo axle adaptors, and it's been six weeks now and I still don't have the ones for my rear hub. Instead I have two sets of adaptors for a front hub and a rear wheel that's held onto the bike by the skin of its teeth. It's been fine for groceries so far, but I'm not going to put a passenger back there until I have the right parts. If I knew it was going to be so arduous getting a part that the distributor lists as in-stock, I would've just planned on brazing in spacers and using a perhaps-stronger hub, like a Chris King, with their 22mm dropout-to-dropout axle that fits in a normal 10mm dropout. On the other hand, it might be been weird using a hub that cost almost as much as the frame.
The ultimate solution would probably involve a thru-axle and asymmetric 150mm dropout spacing like a modern downhill bike. That would give you more strength than the 14mm freewheel hub and lets you fit a modern 9-speed cassette and a disc brake while maintaining a large flange-to-flange distance and not having any dish in the wheel. The problem for Yuba would be getting such a part made at a cost that makes sense in developing nations.