We use an enterprise web filter called Forcepoint (formerly Websense) at my company. Parked domains are blocked by default for standard setups. This is pretty popular software, used by thousands of companies. There are similar alternatives used by many other companies that do the same thing.
If you're pointing your domains to well known parking platforms, there's a good chance that a decent percentage of corporate users never reach your page.
I've recently moved my modestly sized portfolio to Undeveloped and I've seen a fair increase in reported traffic over Bodis. Makes sense considering I always received block pages at work when my domains pointed at Bodis. Same goes for Sedo. I haven't tested Afternic parked pages but I bet they also will be blocked.
I suspect Undeveloped might eventually have the same issue if their pages are ever categorized as "parked domains" by these web content filter companies but for the time being they are loading and not being blocked. The platform doesn't serve ads so that might keep it from ever being categorized as a "parked domain"