5-10 years ago, the GUE Fundamentals course was the only course available where your proper execution of team buoyancy, trim, and communication during drills had to be impeccable in order to pass the course.
In spite of strict written standards, some non-GUE tech instructors (incl. IANTD) might let you pass with weaknesses in any of those things (and others). But it is instructor-dependent. I have seen non-GUE instructors hold people back too. The gap could narrow between agencies, but I suspect differences can still be seen.
Many [non-GUE] instructors don't want to be a meanie and get bad reviews, or just want to get the courses done, etc. Next, please.
The GUE instructor culture says you cannot cut corners, and sells this to trainees as you need this, and it's worth the extra time and money. The strictness is part of what you are paying for--to be tighter in your skills, a better teammate, safer etc.
Frankly I think the GUE spirit--which began as an elite cave diving movement--gets a bit too cavey for open water divers. Like a little sand gets kicked up on one less than perfect back-kick and OMG YOU SILTED THE BEACH WE ARE DEAD NOW
(sorry guys, kinda true though)
So yeah definitely do the GUE courses if you don't think there is any other instructor who is going to be honest (and strict) with you about proper skills. You'll be a pleasure to dive with after that, more skilled, better control, safer, respect rules, excellent communication, sloppiness corrected.
You could also try asking a qualified IANTD TDI SSI PADI person to be extra strict on you (and give them the extra time & money to do so).