The majority of this "free" stuff is pirated and illegal content.
Don't get me wrong, Kodi, the program itself, is not illegal. It's these unauthorized streams from third-party add-ons that are actually violating copyright laws.
A simpler way of putting it is to think of Kodi as a software tool, similar to a web browser. You can access illegal content through your web browser but that doesn't make the web browser itself illegal.
Additionally, these black market "Kodi boxes" do not come with any sort of warranty and are sold as is.
Kodi's developers do not have anything to do with these pre-configured streaming boxes so don't expect any support from them.
Given how these third-party add-ons are not maintained and often break, you may end up with a useless gadget in no time.
Aside from illegal streams, there's also a lot questionable and unsafe content like porn and unauthorized webcam streams you might not want to be seen in your home.
Another serious concern is that if you're not careful, Kodi add-ons can be a source of malicious packages that can wreak havoc on your computer and your home network. There could even be fake add-ons for paid streaming sites that are actually phishing scams.
Additionally, if you want consistently high-quality content, then look elsewhere. Reliable content is hard to track down with third-party Kodi add-ons since they are not regularly maintained. Aside from that, media hosts are over the place, content is unpredictable, and plug-ins often break.
It's not really the most user-friendly and most elegant way to entertain yourself, to say the least. Most of the time, even with official add-ons, you'll spend more time looking for streams that will actually work.
Another security and privacy risk is IP logging. Honey-pot streams can be set up to obtain records of IP addresses accessing illegal content. These records can then be used for evidence if the need for legal action arises.
In fact, internet service providers have also started to send copyright infringement notices to their subscribers, not only to copyrighted movie downloaders but for illegal Kodi streaming activity as well.
So to answer the question, "Should you buy a fully-loaded Kodi box?" - the answer is "NO."
First, Kodi and all the loaded add-ons are free and with the prices they're asking for these days, you are probably getting scammed.
Second, most of the free content that's being advertised with these Kodi boxes are more than likely illegal and you will ultimately be breaking the law.
For better and, more importantly, legal ways to stream, you're better off getting these streaming gadgets that smart cord-cutters use.