The old generation ELTs prior to the introduction of 406Mhz ELTs were, I agree, notoriously unreliable.
However all those that are 406Mhz conform to new specs that were introduced, RTCA DO204 and Eurocae ED62, and spurious signals etc tend to be down to maintenance issues etc.
The type of ELT installed on the Malaysian aircraft is that used on many other Boeing and Airbus aircraft. It is a top of the range model known for the quality of it's construction. Yes, a ditching would certainly provide the de-acceleration forces required to trigger the "g" switch.
Serveral references have been made in the past to the 1992 Air Inter A320 Mont St Odile crash, and the fact that the ELT did not go off in this event. There is an extremely simple explanation as to why no ELT went off, since surprising as it may seem, no Automatic Fixed ELT was installed on that aircraft. Most likely because at the time it was not a mandatory requirement