The following assumes 525 line NTSC and 625 line PAL:
The horizontal scan rates are nearly identical (15,734 Hz for NTSC and 15,625 Hz for PAL), so this is not likely to be a problem. If these differed significantly, then there would be design issues similar to those for multisync computer monitors and this would drive up cost.
The vertical scan are slightly more of a problem with 525 line/60 Hz NTSC and 625 line/50 Hz PAL. But it is a lot easier to design vertical deflection to accommodate a modest variation in rates. TVs could be easily designed or modified to accept either.
The color encoding techniques differ but inexpensive ICs exist that can deal with either standard. In fact, many are programmable to do either with a jumper and slight modifications to the external components.
Displaying a monochrome - B/W - picture on the other kind of set is usually possible if the set has a vertical hold control or enough vertical range. Modifying the chroma circuitry is more complicated but it should be possible to substitute a second IC and patch it into the existing video chain.
As far as commercial multisystem TVs are concerned, the real reason we do not see many of these (at least in the U.S.) is lack of demand. They are available if you look hard enough and are willing to pay a premium. They are readily available on the international market.