I've only had good luck with Uber on longer fares, 200B+. It's really not worth the premium in my opinion for shorter fares. Also, unlike GrabTaxi, Uber has a much shorter list of drivers so finding a ride can be difficult at peak times. I've tried using GrabTaxi late at night in the Sukhumvit area without success. There's tons of tourists looking for a ride and are willing to be gouged because they're either drunk or have company they're anxious to get down with. I asked for GrabTaxi to pick me up from the door entrance of the Westin Grande although I wasn't staying there. Taxi never made and the call center only apologized and offered to send another taxi. I was quoted 10 minutes but the second driver didn't arrive either. Don't bother with these services if out late at night in the Sukhumvit/Silom area because they'll be a no show based on my few attempts.
You're right with your last sentence. The small surcharge still nets less than picking up a fare without using the meter. Unless they hold drivers accountable with future fares, these services will not take hold in this market.