I currently have four unmodified Pentax digital meters. The oldest one has gone through hell and is held together with electrical tape. The latest one is like new and kept in a drawer as a reference to the other ones, with its battery temporarily installed only for such occasions. All of them read identically over their full range. I also once had a Minolta spotmeter which itself read identically to the Pentax meters. A few times, when a deviance was detected, or I outright dropped a meter in a stream, it was sent in to Quality Light Metric for recalibration. That was an average of one meter per ten years! - and even then the odd duck was only slightly off from the others.
One of the problems with the Z VI modified meters is that their supplemental internal filters can fade; and the added flare reducing internal paint can flake off. You can still send them to Richard Ritter for a tuneup. I myself have taken the alleged benefits of that meter conversion with a grain of salt; but there are even Hollywood cinematographers who swear by them, while others in the same trade stick with the unmodified ones.
Fred Picker was an interesting salesman, patent medicine wagon n' all. He did have some really good products, but also some questionable ones. Technique-wise, I'm quite skeptical of his advice to taking meter readings through b&w contrast filters, versus applying proven filter factors after a straight reading. He alleged his super-duper improved meter solved that dilemma. But how can that be, since it was optimized for one specific film, namely Tri-X 320? - and even pan films differ from one another in specific spectral sensitivity. For example, TMax 100 and Delta 100 are alleged to be comparable films; but I know from careful testing that their filter factors differ - half a stop in the case of a medium green filter. Then you've got orthopan films like Acros, which differ even more. And heaven help you with true ortho film.