They're built about as well as any mid-level acoustic. I've never heard of any of them pulling the bridge up and swelling the top like a cheap 12-string, so they must be fairly stout. The earlier ones were eye candy and were pretty inexpensive in the market, so they sold like hotcakes, lots of so-so fingerboard inlay, but nice looking.
An 'acoustic bass guitar' with a scale less than a cello is a nice wish, at least, and especially in a five-string. The only one I ever heard that really, really worked was the Ribbecke-Hafling, but they're pretty rare and NOT cheap, but then they are not cheaply built. Fretless, and mic'd right, it really could very nearly pass for an upright, mind-blowing.
A five string MK is on my 'one of these days' list. Plugged in, they really need an outboard preamp, or at least going into a very good board to avoid that 'clavinet on steroids' piezo clack, if they're not buffered in their onboard electronics.
Joey