I have spent a lot of hours on the water trolling u-rigs for stripers. I have occasionally caught spots and largemouth. Boat speed is usually around 2.8 to 3.2 mph done with the big motor. Line out varies depending on how deep you want the rigs to go. I have hooked up as short as 35 feet and as long as 150 feet. In most states there's no limit to the amount of hooks allowed on a rig, however, TN limits you to one hook. Most catches are made on the center jig. Even when multiple hook-ups occur there's always a fish on the center jig.
I used the u-rigs primarily has a fish locating tool for tournament pre-fishing. U-rigs are good for putting numbers in the boat, however, for striper fishing the larger fish typically come on big live bait.
There is a small version of the u-rig made for crappie fishing, (lighter wire, shorter arms, etc,) Can't say that I've ever tried it.
The most productive way I've found for putting big numbers of crappie in the boat starts with a good fish finder and ends with 8' cast net with 1" square mesh, but then that's illegal in most states too.