What if a counterfeit check had no mistakes or errors?

You can order pre-printed blank checks for some dollars, on the real paper; you tell them which account number to print on them.
There is nothing magical to it, many small businesses do that all the time. Just order a hundred checks with your neighbor's account number on it, and use them.
(That would be illegal, of course!)

The US check system is antiquated and based on honor and ethics. It doesn't take much effort to abuse it, unfortunately.

Creating the checks is not difficult at all.

What is supposed to stop this is that the entity that accepts the check is ultimately liable if the check is no good. Therefore, it is their responsibility to try to ensure that the person giving the check is authorized to do so.

If the crook gives the check to a merchant who accepts it, the merchant will ultimately need to pay back the money. This is why you used to see stores with names and photos of bad check passers behind the counter, and it is why there are fewer and fewer merchants that accept checks today.

If the crook gives the check to a bank to cash, that bank will need to cover the check. This is why banks usually require people to show ID when they bring in a check, and it is why they charge fees when a non-customer want to cash a check.

The biggest hurdle the crook has is getting someone to accept the check. Once they have, they have ultimately stolen from whoever accepted the check, not the owner of the account. They may get away with it until they are caught.

What is to stop people from doing this like they did my uncle?

Somebody had to look at the thief's ID, which of course could have been fake. The place that cashed the check should be held responsible monetarily. The police have to be notified so that a criminal complaint will exist. Then when they are caught the thief will be held responsible. Which can include restitution, fines, and jail time.

What’s worse is they didn’t deposit it, they straight cashed 5 checks of $3,000.

If they cashed all the checks at the same time for cash, the place they cashed the check should have been suspicious, and in the United States should have reported the transactions. If they went to the same place over a few days that also should have triggered suspicions. Of course multiple locations would have escaped detection.

The way to not lose the money is to frequently check the transactions on all your bank accounts. Banking regulations protect people, but require that mistakes and fraud be reported within a specific amount of time.