Both systems are VISUAL approach systems and it doesn't matter whether you fly an ILS in cloud to say 3 miles and become fully visual either on a T-VASIS or a PAPI the resulting visual angle of approach is the same. So it has nothing to do with coming into the view of the VASIS from an instrument approach or not.
When PAPI was first mooted many years ago, it was argued correctly that PAPI indications were colour dependant and thus wide open to the pink or vague colour difference under certain atmospheric conditions that was mentioned in the early posts. In fact the PAPI is still a very accurate visual aid, providing that it is serviced correctly - and that is the problem.
Like all VASIS types around Australia, once the initial installation is completed and a commissioning flight test by a dedicated aircraft is completed, there are no more scheduled flight tests unless pilots submit a formal complaint with full details of what they saw.
Even then, although airport Maintenance staff may go out to the VASIS and have a look for crook lamps, snakes, long grass or earth movement etc, there will not necessarily be a flight test arranged due to the cost of flying the dedicated navaid test aircraft to wherever.
In the old days when the Department of Civil Aviation Flying Unit F28, F27 or DC3 were on tap, a crew would be despatched to re-test the VASIS at tax payer's expense. VASIS was also subject to routine annual flight tests. Now that a private operator has the contract for navaid calibration work, the tendency is to cut costs to an absolute minimum and therefore the services for a dedicated VASIS flight test by the current contractor is seldom called for.
The upshot of all this is that it is left to pilots to officially notify ATC or the appropriate authority that runs the airport if they perceive a problem. A certain degree of colour blindness is allowed for pilots for purposes of holding an ATPL, but the downside of that is some pilots may experience occasional difficulty in interpreting PAPI signals. It was no problem with T-VASIS unless you were in a gross 1.9 degree undershoot in the red, but then the three fly up lights had already given you advance warning of an impending undershoot.
While I personally have always liked T-VASIS and prefer it to PAPI for several reasons, the fact remains that it is more expensive to maintain. The atmospheric conditions that can give rise to erroneous VASIS light signals also can affect PAPI. Vague pinkish colours rather than bright red lights can cause confusion.