Lets clear up one important issue. Handheld GPS units are not certified for anything.
That means that they are not certified for VFR and not certified for IFR. So these units are not to be used as "the sole source of navigation information".
Handheld GPS units are never going to be certified. It is impossible.
It has nothing to do with the capabilities of the unit or the maps or the database.
Some issues that can not be quantified in a certification process;
1. Antenna positioning. - internal antenna can be subject to screening and with no defined position for the antenna the effect of screening can not be determined. Where a sucker is stuck on the window may not be the same for everyone and every aircraft. Using an external antenna does not get round all of this issue because the connection is made and broken every time the unit is used.
2. Battery life remaining. How long are those AA's going to last from the start of the flight?
3. RF Interference. No problem in aircraft A but aircraft B with a different comm radio uses an internal frequency that interferes with the GPS signal
4. The drop test. Dropping the unit can cause internal damage. Do the same with the aircraft and it will be obvious that there is damamge.
5 Position of unit in cockpit with regard to compass, pilot vision and ease of use.
Finally and this is the one that everyone seems to forgett when they use a handheld GPS for VFR (or IFR) back=up information ;
The unit when placed in the cockpit and it's attachment must comply with the certification requirments regarding G loadings and not become a missile during all manoeuvres within the envelope as well as the accelerations that are required to be allowed for by the certification requirements (accident situation).
Not many people know that it is against the rules to place a handheld GPS on top of the instrument pannel. If due to a sudden movement of the aircraft the GPS unit departs through the windscreen, the first question going to be asked is why did it's attachment not comply with the required standards!!
Check the FAR/EASA standards for the restraint requirements.