I've never tried a xtracycle. I can tell you I don't have trouble balancing my bike while waiting for the stoplight to change. In fact I can do a no-handed "track stand" if I put my left foot on the tow bar! Care must be taken not to put too much weight on the towbar, especially when unloaded, you can flip the trailer that way which is embarrassing.

I'm not qualified to judge, but I can't help thinking a trailer is easier for hauling, although I've seen some impressive pictures and videos of long bikes being ridden with big loads.

The problem with wide loads on a trailer is cornering. Just like long truck, long trailer must make wide turns, even a narrow model like mine. It shouldn't be hard to design a structure to support the table in an upright position. Try to keep it light though, memnoch is correct about the the additional work involved when adding weight to your load.

I predict the engine will get narrower.