SmartTV's have about the same security of other IoT devices like network-capable DVD players, IP cameras, routers, printers etc. This means: while there are some vendors which actually care about security there are others which don't. Specifically with the cheaper devices the chance is higher that the device is insecure but spending lots of money does not guarantee security either.
And if such device gets compromised it can both be a danger for the local network as for the rest of the internet. For one security issues might allow the device to be taken over from outside so that it can be used as part of a botnet to attack other systems, to spread spam and malware etc. But one notably difference between a SmartTV in a hotel and others IoT devices there like IP cameras, routers or printers is that the customer is actually expected to interact with the device and that the customer has physical access to it. If the device is in the same network as sensitive systems it might be used to attack other devices in this network. Putting these potential insecure devices into a different and strictly separated network will mitigate this risk.
It does not matter much what kind of OS is installed on the device. More important is if the software is recent (and not some old Android with known security issues) and if the vendor actually cares about security and provides quickly updates when needed.
For more information about this topic see for example How my TV got infected with ransomware and what you can learn from it or Samsung and Roku Smart TVs Vulnerable to Hacking, Consumer Reports Finds.