how secure is an apple tv unit

The only video output of the Apple TV box is through the HDMI cable.

Video input is possible through AirPlay, which the user can lock down or set wide open.

Internet communication is possible over Wi-Fi or ethernet, which is as secure as your home network, unrelated to the security of the Apple TV itself.

Most internet connectivity is encrypted (SSL, TLS, etc.), but that may depend on each app.

Third party apps do not have access to what other settings and accounts are on the device (sandboxing).

Keeping the system software up to date should help keep it secure if any vulnerabilities are discovered and patched. Apple security updates - Apple Support

There is no documented way to gain access to the Apple TV from the outside, e.g. in order to snoop around, or change things or influence things.

The storage (SSD disk) is encrypted like File Vault, and would be unreadable if removed.

If they already went to the trouble of hacking your home network, which they would need to do before they can get to the Apple TV, the last thing they'd want to get into is the Apple TV. Computers, and mobile devices on the network would be a more logical target.

There's really nothing of interest on an Apple TV for hackers. What do your expect them to get from it really? The fact that you maybe binged watched a Netflix series the last 3 days?

The only personal information on an Apple TV is your Apple ID, and even then, its only your email address. Sensitive data like your password and payment methods are stored encrypted on Apple's servers not directly on the Apple TV.

Hello,

It is physically impossible for an Apple TV to be remotely controlled by a 3rd party.