tl;dr: Mobile devices and designer watches with (RF) emergency beacons don't work without line-of-sight to some kind of receiver. Most types of RF communication signals require a close approximation of line-of-sight between the transmitter and receiver to work.
All GPS receivers require line-of-sight to fix their position. However, this line of sight is up (because the satellites are in orbit,) which is usually not obstructed unless you're inside a building, under water, under a bridge, under ground, etc. Any GPS receiver will stop working under those circumstances. Airplanes are very rarely in situations where their GPS receivers don't have a working signal, except when they're in a hangar. Since they're normally above ground and outside, they normally have no problem getting a GPS signal.
Where the problem comes in is transmitting their location back to someone who's listening for it. This requires either radio communication to some ground-based system (or another aircraft,) such as the ADS-B receivers mentioned in the question. (Roughly) line of sight is required for this. The same is true of mobile devices. They also must be in range (which is often less than line-of-sight) of a receiver that is listening for their location transmission. In the case of cell phones, these receivers will just be cell towers. This is why, for example, we can't just track down the position of some crew or passenger's phone when looking for a boat or aircraft that has gone missing at sea (where there aren't any cell towers in range.)
Note that radio-frequency signals (such as ADS-B or cellular communications) can penetrate some objects, but this ability is very limited, especially as you move to higher frequencies. Thick, dense objects (like concrete parking garages, overpasses, and tunnel roofs) will reduce (attenuate) the signal faster and conductive objects (such as water or metals like copper or aluminum) will nearly eliminate the signal with only rather thin layers. Also, in general, higher-frequency RF signals will be attenuated much more quickly than lower-frequency ones. This is why your home Wi-Fi or a cell phone works through perhaps a few walls, but the new 60 GHz wireless networks wouldn't.
In the case of cell phones, the broadcast range is also intentionally limited so that more towers using the same channel can be packed into a smaller area to serve more people. If they didn't do this, there wouldn't be enough bandwidth to handle all of the cellular communication needs in densely-populated areas.
The Earth's surface happens to be composed primary of rather dense soil and water, so radio signals don't travel through the Earth much at all. In general, any form of radio communication must either have no land or water between the transmitting and receiving antennas or else cause the signal to reflect off of higher layers of the atmosphere or curve around the Earth. Neither of those things works well in the frequency bands used by cell phones. It does work at some of the lower frequencies used by amateur radio operators and such, though.
Communications systems that need to work far away from land or otherwise in remote places where receivers can't reasonably be located within line-of-sight range on the surface will normally use satellites. This is because Earth is not between you and a satellite that is above you and, thus, isn't blocking your signal. Even connections to satellites don't normally work when the satellite is below the horizon (again, due to the Earth attenuating the signal,) but satellite communication systems are normally set up in constellations that are designed to keep at least a certain number of satellites above the horizon in coverage areas at all times or else are set up in geostationary orbit where they will always remain directly above the same surface position.