They are ok. 2 roller pumps have a couple problems if not set up correctly. They can stop in a position that forms a siphon. Or if they dont form the siphon problem the line can drain back and thus the next dosing cycle will not be as accurate. Both problems can be solved with added inline 1 way check valves.
profilux dosing pumps are good but not always easy to find! With 3 rollers on the tubing helps to insure the above referenced problems dont occur, as one roller is always pinching the tubing