Hi
You may be worrying too much. The reason why IoT devices are often on a guest network restricted from your normal LAN is:
1) Performance: Lots of IoT devices can end up being quite chatty with broadcasting packets, so having them on their own network stops those broadcasts arriving everywhere else and potentially slowing things down. Typically though this isn't an issue for a home network and many IoT gadgets will need to be on the same network to work correctly or be discover-able from your other devices like tablet or phone.
2) Security: Some IoT devices have had a bad press for being able to be hacked into and often be made to reveal the Wi-Fi password, someone potentially could then sit outside your house and join your Wi-Fi network and do things that probably isn't legal. If they can only join a locked down Guest network you restrict them from accessing your more important devices. Has this actually happened to anyone? A lot of things need to be lined up in a row for this to happen.
Given 99.9% of people don't bother doing any of the above and don't have any issues, with a large percentage just using silly default passwords or unsecured Wi-Fi, all you need to do is have strong passwords and keep things up to date and any hackers will move on to far easier targets.
Regards
Phil
