Now that the good controller companies have gone out of business, because people would only buy cheap, only the factory controllers work by responding to the drivers foot pressure. The after-market guess-work controllers are not something I would use.
There never was any kind of rule about connecting to the hydraulic system to add a real controller. Only if an old add-on controller required too much brake fluid to operate would there be a problem. The excellent controllers from Brakesmart and MaxBrake used a sensor that required no fluid. For those that complained about adding a simple tee, I guess they never replaced brake hoses, lines, master cylinders or wheel cylinders. Cheap and easy was the only thing that mattered.