Where did I say it was off, or repeatedly off? It's repeatedly accurate for me. The package is quite good for the price.
There's really only one way to define accuracy here. One is to measure color patches with the product and compare that to a known, higher grade reference device.
The other thing you can do is repeat the measurements with the same unit and compare the differences, that tells you nothing about the accuracy of the data, only the repeatability.
And lastly, unless you measure the same target with two different instruments and build two profiles using the host software, all you can say is, you're happy with the profiles from the one product (you can't discuss accuracy).
Why would a proflier have to be upgraded for a paper if it's able to read the reflective densities and colors?
Here's what I suspect is going on, its only a guess based on what I've read about this product, what happens when people get poor results and when they get newer software to adjust/compensate/recognize this paper. I suspect this device isn't really anything much more than a colorimeter, perhaps with more filters but not a true Spectrophotometer (its not reading spectral data) and thus, it has to make a bunch of assumptions about what its measuring. If the assumptions based on the software are correct, you get a decent profile, albeit with tweaking and printing using the provided sliders (another reason I question what's going on under the hood). If the assumptions are off, you get a crap profiles.
There's no question in my mind that, based on those users reporting both good and poor results, there's something not totally well defined in this process. This is NOT my experience with other profiling packages using true Spectrophotometers (Moanco, GretagMacbeth, Praxisoft, Agfa, Fuji, etc).
If you read all the way to the end of the comparison article in the links I posted, you'll find that it did quite well, unless you are trying to make a perfect print of a grainger rainbow, an artificial construct.
Not at all. As I've said, its hit or miss. The links you provide do not address all papers and printers and I can if I had to, find plenty more posts where users reported problems.
Your own book says that several of the better profiler packages, such as Eye-one XT, Fuji's Colourkit, Monaco Proflier Platinum (w/sliders), etc. have profile editing and adjusting capabilities. Is this a sign of inadequacy or of flexibility?
I also say they are and should rarely be used, that if you start with a poor profile, don't even start to think about editing it. If you want to do cross rendering, its useful. If you want to do very minor tweaks, long after printing lots of images, fine. This isn't how the CV editing tools are presented to the user. 99% of the time, you should never need to touch any such sliders if the profile is of good quality out the gate. And you'll notice that ALL the products you name provide a secondary editing process, not something you'd ever address while building the profile itself.