I definitely agree, but the ADS-B coverage between Malaysia and Vietnam just before and after Flight 370 are faultless. Flight 370 was in trail behind MAS52, which had complete ADS-B coverage all the way to Vietnam, and another flight followed with no problems in coverage either. Is it possible they are sourcing their data from ARINC or one of the ATC centers there?
FR24 coverage for yellow airplanes is from their own receiver fleet and/or private receivers coupled to their network.
Coverage of MAS370 at the last positon was from "F-WMKC1" station, which suggests it is located at/near Kota Bharu. This stations covers the bay until close to Vietnam. Another station in the area is "F-VVTS1", but it seems not to reach the particular area. Kota Bharu is about 10 min flying time from the last position, roughly 80 NM. No problem for any well located ADS-B receiver of any make.