If a tech says it's going to take 3-4+ hours, I'd say he's ill-equipped, and thus doesn't know what he's doing. Being ill-equipped is probably more dangerous than a diyer being properly equipped.
Using the proper tools when working with springs is very safe, that's why you're using it, and that's why they cost so much. If you're injured while using a spring compressor, it's probably because the tool failed, and in which case it's not your fault to begin with.
But purchasing (or renting) the proper tool for the job is incredibly important to prevent injury. However it rewards you with flexibility in doing the project on your own time, at time savings, and if need be, money savings since you can repeat the procedure over and over until you get it right.