I'm in PNH now and can confirm as well that Grab works without cash if you already have a foreign payment method saved in the app. The pick-up point at PNH arrivals is not far, easy to get to even with luggage.
Grab Car has been an older Toyota Prius every time, Grab SUV is not much more expensive for something a bit more spacious like an old Lexus SUV.
Although places you tend to go to are often not far at all, the city is not very walkable in terms of sidewalk condition, dirt and smell. I love to walk and in other cities I'd opt for walking a few km, but here I find myself using Grab even for a 1km trip. The price is less than 2 USD and walking is just not very pleasant. Not due to heat, it's actually not too stuffy and the air is not bad, it's just tedious to walk, you constantly have to look where you step.
Here are example options at 8am on a Saturday from the Hyatt Regency to the airport:
Grab options and prices in Phnom Penh