this is something that all NAS companies do. Obviously they want to cover the cost of development for the surveillance app and also make some profit.
But can they do that, they are using Linux kernel and probably even other open source apps which fall under GPL license which means that if you requested the source code they would have to send it to you, so setting the limit to IP cameras unless you pay for more would be like OpenMediaVault setting the limit on how many drives can you use unless you pay for some kind of license, right?
On the review, I have to say that 4+ bay NAS devices really have to have very little downsides because if ease of use, dimensions, low power, DAS function don't matter to the user it is cheaper to set up PC NAS server with better specifications.
This NAS device could have eSATA
p which offers power so connecting external HDD would be easier but it is fair to say that not many devices use it and that the main use for that port is expansion unit.