USB tethering to laptop safe?

I surely hope so because that's the way we do it when on vacation(s).

When ethered to a laptop or a desktop, they are only as safe as the cellphone your using, namely, if someone can hack your phone, they can then access your computer.

Define "secure," and how much you think "they're" out to get you. Government could tempest the wired connection and if you think the cellular networks are free from gov't snooping, nope.

This day and age who knows what's safe anymore. Using your phone as a hotspot is much safer then using public WiFi. A more pressing issue is Bluetooth if you Re really concerned about security as there are known vulnerabilities. Someone would really want to hack you though.

USB Tethering is secure between your phone and your laptop but really not significantly more than using the wifi hotspot with a password. Someone can tell if the wifi hotspot is active but can't break in and do anything with it without some high end hacking skills.

Far more likely is someone getting access from a website you access and download from.

Assuming you mean a sniffer that's not on your laptop, then no, it cannot find the connection. USB tethering is a wired connection, not wireless, so other people cannot see it.

Now a hotspot is another type of connection that can be seen (although it can be made secure) because it's wireless. But you said it's USB tethering so no, you're good.