I put a 7-speed Shimano Nexus on the 1973 Schwinn Twinn my girlfriend and I recently bought. I have a lot of experience with the Nexus. I worked on a Moon Buggy project at Arizona State; we built human powered, tandem, 3 or 4 wheel, folding, off-road recumbents and used the Nexus as transmissions.
The gear range on the Nexus is just about perfect for the Twinn, the low gears make climbing bearable/possible and you can really get it going in the high gears. The other cool thing is that you can shift even when you are stopped and not pedaling.
The Nexus should be available with a thumb shifter or a grip shift. The thumb shifter is much better for the Schwinn bars but they are harder to find.
The roller brake is good considering the weight of the bike but it has to be set up correctly. It is an improvement over the stock coaster brake. It has a mushy lever feel but that seems to be normal for this brake. Also, I'm using the stock brake lever since I like the vintage look (and parking brake button), different levers do improve it some.
On the Twinn I did have to spread the rear dropouts to 130mm but it is pretty easy with the right tool.
I'm using the 21 tooth cog, it is possible to get other sizes if you want a slightly higher or lower gear range. The cogs are dished and can be flipped over to get the chainline right.
As for reliability I haven't had too many problems with them. On the Moon Buggies we over torqued them sometimes. But we were using significantly lower gears, the Buggies were heaver, and on some of them there was one Nexus for both drivers. This would only damage the drive cog which is easily replaced.
Getting it setup is pretty easy, the cog is held on by a snap ring, and over this there are three pieces that hold the cable and the cable nut, and rotate to shift gears. This cable nut has to be attached a precise distance from the end of the cable. You then use the barrel adjuster to line up two red marks when you are in 4th gear. The roller brake is bolted on the other side and there is an arm that attaches to the chainstay.
If you have any more questions let me know. I can post some pictures also if you want.