GRAB: how reliable from NAIA to destinations outside Manila?

Why not ask the hotel you will be staying. They might provide or advice for you

In addition. Filipinos are nice and good by nature. We are hospitable. There are few to consider some might get advantage of you because you are foreigner. Grab is okay and safe for you not to mention the cost of travel. Not sure of that.

Safe in a sense you have the drivers picture and record/history on grab apps.. etc you know..

But generally Filipinos are nice and good

Check out the buses.

It is better to check the bus. Grab to Tagaytay is surely a lot more expensive than bus. In additions, let's say the driver is based in Metro Manila, thats why he's driving around metro manila. If you book a trip to Tagaytay, he will drive to Tagaytay and but if there is no guest going back to Metro Manila, meaning he gotta drive a long way home with no income for the trip coming back. If it was me, I wouldn't take that booking.

If for the convenience, find a Grab or other vehicle service. You dont have to worry the way back of the service. Thats how Grab works

If money/cost issue then take a bus.
However you know will be needing a lot of transfer. There is no direct bus from Airport to Tagaytay then when you reach Tagaytay if the Bus will not passby to your hotel/accomodation you have to wait or look for another vehicle to get you in your hotel/accomodation.

To cut it short ask your hotel/accomodation, whatever your reason

Sometimes the airport (yellow taxi) drivers ask passengers if they are in town for a few days, where they are going, etc. Some hire themselves as driver/guide. I was in Manila earlier this year and went to Tagaytay for a couple of days and I hired a driver I had met on a previous trip. He drove me to Tagaytay and picked me up when I was ready to come back a couple of days later. The trip did not originate at the airport but I was in Pasay. I actually found 2 drivers who were ready to drive me there. One wanted 1300 P but he offered this after I had already agreed to pay another driver 1500 P. Don't know when you're going - it's already nearly mid July so maybe this is too late for you but I hope it gives you an idea of the cost.