Poor results

I took my car to car service city( Jet park ) for major service. After I have done my service within two weeks my car started giving me problem with starter and speedometer. I went again to explain the matter and they adviced that my car has problem with sensors then I asked them why they did to pick the problem for the 1st time and they said this can happen any time. The report they gave me never indicated the issue when I collected my car. My question is are they using qualified techs on side or they just hire people. I regret for taking my car there. I would not recommend anyone top take his/her car to that place.