I don't trust what the companies that market them because whenever money is involved statistics can be quite biased. What are the limitations of them, and how safe am I using one instead of getting a vehicle inspected by a mechanic.
The main issue is that to do a thorough diagnostic takes the skill and knowledge of how to interpret live and recorded data. Looking for codes is just the tip of the iceberg.
It depends on what product we're talking about. This scanner below fits in your pocket and you can get quick results from your smartphone.
There are also others out there that range from a few hundred bucks to a $5,000 tool that will analyze all data.
That's like asking, "how reliable are cars?" It depends who makes them and how much you spend.
The ones who makes tools aimed for mechanics have to be good.
