In my view the stuff that makes a dodgy pool builder dodgy is when he uses cheap subbies or non-qualified staff for all the non-pool parts of the build. This is things like the paving, the electrical, the fencing, the pipe work etc etc
Talk to the builders and ask questions around this stuff to get a feel for whether they use good labour or the cheap labour or do stuff themselves that they shouldn't.
My house had a pool when I moved in. I soon noticed some 3-core 240v White cable not in conduit near the pumps, just laying on the ground. I pointed it out to a sparky one day and he said my whole pool area electrical was obviously done by a builder "he could just tell by looking at it". The sparky fixed it all up but he said this was very common with pools.
And as someone said above don't go the cheapest as they are cheap for a reason.