Almost all inwall speakers are direct descendants of car speakers...open-back plates. The singular advantage is they are inexpensive. The disadvantages are myriad.
Any real loudspeaker needs an enclosure, and not an arbitrary wall space to serve as a baffle. An optional "back box" isn't any better, because it changes the Q of the speaker system, and it's usually flimsy.
Believe me, I've avoided this subject here because Triad is the only company that manufactures a complete line of high end custom speakers that actually have fully engineered and extensively braced enclosures, and I didn't want to crow. The sonic advantages of an enclosure are obvious, but just as important is the fact that an overbuilt, solid cabinet will not allow back waves to bleed through the wall into adjacent rooms. Ask any Triad dealer, and you'll get a bit of a smug smile...
Someone please tell me how open back plate speakers can have predictable results when every wall cavity is different. And when the fully enclosed inwall speaker uses drivers from Seas, scan-speak, Vifa, or Peerless, the results are even better. Triad inwalls start at $700 a pair, and a high-zoot inwall home theater speaker setup can exceed $20,000, and all products are made in batch sizes of one, to order. You can actually integrate high-end sound into a room without big, obtrusive boxes. (Of course, we also make big obtrusive boxes...)