Pex is good but can be installed wrong so just check the connections. There is nothing that you can not repair and no the plumbing is not at all that difficult. Just start slow and check the connections first, start at the fresh water tank then work to the first tee. Stop at that point and go to the city water inlet and check from there to the same point you stopped at from the water tank. If that is all good then easy points (kitchen sink) then water heater and last bathroom. I hope that this is not too confusing as I am only half awake and will reread this later!
Gotta get busy! Have a great day! Now where did I put those revits?