You are right and wrong to some extent IMHO (and this is really opinion). Some opinions are based on more experience then others, but as I stated above, there is no one right answer. In fact, when people try to asset (not saying you are doing this), that one dyno IS the perfect solution, I laugh. They all have pros and cons, they are all a bit right and a bit wrong...
I tried to be clear on this, but if you read what I wrote, you will see that I qualified "more accurate" as to ENGINE, but not car.
The Dyno-pack does not measure the power at the wheels - which in reality is where your car puts the power down (as opposed to the hubs). So yes, if you are measureing ENGINE only, then the dyno-pack may be a more accurate tool.
However, in the real world, we use 4 wheels to connect the car to the road, and any rotational issues that the wheels add and the drive-line add are VERY real. So if you are attempting to measure the output of the CAR, not just the engine, then a dyno that runs on a roller is superior.
As to your "one tool is more accurate" comment - you are right. But my retort, is "how do you calibrate that to know which tool that is?" With any dyno, you simply take a baseline, then mathematically calculate the loss factor, then use that factor moving forward.
One step further is the assertion that if a dyno is not emulating the exact conditions your car is really seeing when driving (ie: all 4 wheels on the road), then that is a less accurate tool
The dyno-jet does have an option to add load, but load is only an option you use for tuning, not for power runs, or "shootout" mode as per the dyno-pack.
The best dyno you can use really is a Superflow. The superflow has the ability to actualy simulate roads or tracks with varying load and allows you to monitor many channels of data. This is what most NASCAR teams use as well as other very high-end race builders.
Hope that helps to clear up what I was trying to say. If not, I will try again (I am working on no sleep! lol)
This is a great discussion to have, so if you have any additional questions, please feel free to ask. As I stated above, there are as many ideas and beliefs on this topic as there are options, and there is not a right or wrong answer, it is really a matter of opinion.