Regarding the relative amount of reported lock-ups here in the forum, I would say: Yes, there is a systematic quality problem with the Z9.
Whatever it is, semiconductor technology will always be the weak point of modern cameras. As today's cameras become increasingly powerful, I wouldn't be surprized by an increased amount of reported issues.
I think we should stop blaming memory cards, batteries, firmwares etc... It is what it is. Every lockup is one too much. It's up to Nikon to find out.
The good thing is: The more lockups, the sooner Nikon will adress the issue and eventually release a product advisory.
So, the best thing would be probably to report any lockup to Nikon immediately.
For now, I have cancelled my efforts to get a Z9 and I will stick to my beloved D850 (which might lockup, too, but never did). I simply cannot afford a spare camera.
Unfortunately in today's world companies, politicians, institutions what ever react quicker to chatter - complaints found on social media, forums and the internet generally.
The more chatter the sooner things get attention and are dealt with seriously.
The trust and wait approach doesn't always work, as companies monitor the internet forums and chats and don't act quickly unless they see they have to, in this case with Z9 its serious....especially if your a victim, regardless of the numbers.
ie:
A) if on a expensive paying job or expensive international trip,
B) very expensive if its out of warranty.
C) effects resale if left on going and watered down as trust sells.
Loose a Z9 customer and they loose 5 lens sales, collateral damage ? till they fix it ?
The issue may be small in numbers for now, the issue has been running for years with many previous models and different brands according to the web history, just this one with the Z9 is more serious again if it bricks on you and costs you a job, trip or effects adversely resale as its hard to sell out of warranty with a stigma.
The problem is like a bush fire, it needs to be dealt with fast and properly before it gets out of control, the issue is now 11 months old and getting worse ? or is it ? or just the start of a bigger issue, hence one assumes its possibly more a design/software issue ? who cares its a fix that is needed fast and once and for all.
If it was a small niggling issue we tend to tolerate it and hope it gets addressed, this Russian Roulette or un-lucky dip lock up issue then Brick is a whole different game.
Out of proportion one asks, not if your a Victim.
Many large companies can be very arrogant in addressing issues.
Nikon is a multi Billion organization, it deserves no compassion or sympathy at this level as the issue has been running for a while, it has the money and power to do things properly and quickly, why cant Nikon do what their competitor successfully can,
Only an opinion