Are there Disadvantages of in wall Speakers...

theorhetically, there need be no loss of performance and IF done
right [few are] you can actually get sl;ightly superior performance
in some cases, since the speaker becomes an infinite baffle with NO
reflection whatsoever from back wave bouncing between the speaker and
the wall. this can actually improve bass to some extent.

Additionally the bass becomes half space loaded instead of quarter
space loaded and thus you do not experience the usual 6dB of
diffraction loss normally begins at about 250 Hz/middle C and falls
off. you will only see about 3 db's of bass fall off. There is a
good discussion of this with graphs on True Audio's web site:
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Full crisp, tight discipline punchy low bass will continue to be a
funtion of getting a good powered sub in the corner to take advantage
of the full steradian loading of the low frequencies. You continue
to need to cross the mains over to a POWERED sub, as befor, . to get
tight punchy bass below about 100 Hz or so.

The above is IF it is all executed perfectly. In practice:
1. if you put crummy drivers with a crummy crossovers in the wall,
you get the same crummy sound you would get if they were in a box.
2. Too, if the wall AND its cavity is not fully braced, damped and
reinforced and ideall decouipled from the driver, it makes a really
terrible box, which will resonate like hell. Thin dry wall makes
lousiest of boxes. The best speakers, Theil, Hales, etc, have heavy
front baffles often 2-3 inches thick!! with side walls of 1 inch
thinkeness or more--all from from MDF, which is heavily braced and
damped. Compare this to 1/2 inch gyspum. The difference is like
brick compared to cardboard.
3. You need to make sure the in-walls are angled with their sweet
spots, and particularly their tweeters for maximum coverage of the
listening area and they should be at ear level for listeners, just as
will box speakers.

The other space related anomolies, side wall, floor and ceiling
bounce remain the same, as do standing waves and room resonances.
You only get a tiny bit of improvement over the boom, bloat,
tubbiness in the bass over in-room speakers, this can be real and
useful if done right.

There is one other savings. If you figure that your main speakers
take up 4 sq ft of space themselves, with another, say 20 ft of space
around them, well, at roughly $100 sq ft, you pay a hidden,
"opportuntity" cost as the economists call these, of roughly $2,500
in lost utility for that space in the room that becomes dedicated to
the speakers.