Has anyone found/done any real-world testing?
For example NSS labs does various tests. But since it is their business to help others decide what to buy the detailed test results are only available for money. Some vendors though provide the details on their own website - but of course only if these are good results.
Do the one wire products reliably filter all packets to/from all devices, or do they miss large chunks of traffic?
There is always a balance between speed, price and depth and quality of inspection. This means these firewalls can be bypassed and often in trivial ways (disclaimer: this is my own research) as NSSLabs shows too (which used my evasion tool). And while many of these reported problems have been fixed there are several unreported which still work.
But in general you can say that you get what you pay for. The cheaper a device for a specific throughput is the less analysis it does, which means it also protects you less. But none of the devices offers full protection. And no matter what some claim, I don't believe that any of the vendors has the secret "patent-pending" "military-grade" sauce which makes it possible to provide the same level of security and performance for a significantly lower price than all the others.
Do these things really protect any better than conventional routers?
Definitely. A router does not offer much protection in the first place. These NGFW analyze at the application level, check the reputation of sites you visit, analyze mail ... - nothing of this is done by a router.
Are they more frequently patched for vulnerabilities?
Some vendors have obviously more problems writing secure code while others have less problems. Just look at the reported vulnerabilities before you buy something.
But all of this does not mean that you need to buy the most expensive device to protect your network. What you actually need depends on your specific risk, which depends on what you protect but also how you protect it already. If all your sensitive data are offline than you don't need an NGFW but likely a better lock or a safe or a surveillance system since the main thing you have to worry about are physical attacks.
And even if the data are on networked systems it might be better to have separated networks depending on the sensitivity of the assets, use microsegmentation inside the network or your zero-trust concepts like BeyondCorp to protect your data. Depending on your specific use case and risk they might protect you better for the same costs than some next generation perimeter firewall.
In contrast to what especially the cheaper vendors tell you: security is not as simple as putting some fancy device on the network and be fully secure.