Well, it's all relative; depends on what you're comparing it too. My older brother at one time sold Rollei MF SLR gear, among other professional lines. He'd demonstrate the softness of the shutter by placing a camera on a solid table, setting a dime ON EDGE atop it, then would use a cable release to trip the shutter. The dime didn't even move. And he owned a couple of SL66 kits himself. If you had done the same thing with a P67, the dime would have landed in the next county. But he preferred the ergonomics of my Pentax 67, and once borrowed it an entire decade, and even got crisper shots than with his Rollei, perhaps due to the more intuitive viewfinder. I printed some of those shots, and don't think MF work even got much tighter with respect to the available films of that decade.
As far as the big handle goes, I never routinely used one, nor did my brother. He alway used a Kenro gyro mounted to the camera when shooting from a chopper or airplane. He had a small plane flying license. The only reason I have one of those handles is for sake of not clumsily dropping the camera with a big heavy 300EDIF lens on it. I already mentioned how I use a large Ries wooden tripod for that specific application. But I have successfully shot that big lens without tripod, rifle-style, by rolling up a coat and placing it atop a car roof or fence railing, and resting the camera on that for sake of a relatively high shutter speed. Otherwise, at least in the way I use the system, I have seen zero practical evidence of shutter interference with sharpness, and I print about as crisp as it gets.
I do know a specialist in extreme telephoto work who mentioned a problem with the shutter. That was way back. He would know, having been both a Pentax and Celestron dealer at the time. But in more recent years, the combination of P67's and long EDIF lenses was popular for extremely nitpicky widefield astrophotography (comet hunters, etc), which is about as demanding an application one could think of. There were even vac back modifications for 220 film available for the P67 for sake of optimal image plane sharpness. But those kinds of folks were also using clock-drive "tripods" hauled in trailers and costing 50K or more, or sometimes in conjunction with little private observatories costing several hundred K. It's all out there in terms of their own forum discussions. But instantaneous opening and closing is admittedly a different ballgame than hours-long exposures. The only advantage the later P67 ii had was the fact the mirror lockup didn't drain the battery over long exposures like the previous MLU units, or that handful of early lenses not yet up to present expectations.
My own experience with the big 300EDIF for terrestrial work fully complements just how superb that particular series of lenses is - the Mercedes Benz of MF telephotos as far as I'm concerned. But you could hypothetically argue that the sheer mass of those big lenses helps to dampen any shutter issue; and you might be right. But I get equally crisp results with their later wide-angle lenses using MLU. The old advice was don't shoot them between f/60 and f/8 where the vibration is allegedly worst; but based on thousand of my own shots, I find no practical visual evidence whatsoever for that old wives tale. But I've never owned a pre-MLU unit.
Nice hearing from you and your own opinion. Now that I'm retired, I've gotten into MF photography a lot more than before. I basically concentrated on LF sheet film work for four intervening decades. And for handheld applications, I mainly use my Fuji 6X9 RF's instead of the P67. The last two weeks I carried both systems in my pack, with the Pentax mainly being used for telephoto shots. Hoping to print a couple of those later this week.