If anyone worries about whether their design is correct or not it's because they've never treated PII with the respect it deserves.Maybe that should be true, but it isn't.
As a personal example, let's look at one of my own businesses that is not even remotely in the aggressive advertising or other data mining space. The only PII we keep is basic records of how our systems are used, who our customers are, what financial things happened, email correspondence, and so on. Nothing even remotely controversial. Nothing that more or less any online business wouldn't have to keep, often due to legal requirements.
And yet in at least one case we're still worried, because having read the actual GDPR, it is far from clear that things like our backup and logging systems aren't technically in violation. We don't know exactly how we would react if a customer did suddenly request deletion of everything we held on them, because we don't know exactly what our legal obligations would be in that situation.
Clearly we couldn't just delete everything, because we'd be required to keep a lot of that data for legal and accounting reasons. So if nothing else, we'd then have to spend hundreds or even thousands of pounds on taking expert legal advice, and then potentially hundreds or thousands more to update our systems to remove whatever we had to remove.
This is a side business run by a few people, part-time, like many other small or microbusinesses all over Europe. We're not dealing with anything like the scale of personal data of the Facebooks and Googles of the world. Apparently our carefully designed systems, which only collect a reasonable amount of data for legitimate purposes and take care to handle it securely, are already doing significantly better than some of the biggest data hoarding organisations in the world.
And yet, the effort and overheads required to address GDPR could pose an existential threat to the business. Tell me how that isn't disproportionate and how it's all clear and been thought through.