Last year, I bought a boost of those guys in order to boost me a single rank. They did so successfully, the booster won all his games, after that I continued to play on my account, continuing the win streak he (the booster) started. Ranking up another 3 ranks. A week later, season ends, I'm getting an alert by Riot Games that my account rank got reset due to boosting activities. So much to their "safe boosts".
Shortly said, don't trust them. The only safe boost you'll ever be able to have is if your Master buddy willingly duo smurfs with you.