Do iPhone 6s purchased through ATT have different hardware?

I have been told that an iPhone 6s purchased through ATT may have some hardware incompatibilities with other carriers even if the phone has been unlocked by ATT and the carrier is GSM. Two carriers have stated this.

Has anyone heard of a difference in hardware?

Is a 6s a 6s regardless of where it was purchased?

I want to be able to take this phone to Europe and use a SIM card from there which I have done with other phones in the past. Now I am a little Leary and don't want to get over there only to find my phone won't work with their SIM.

Thanks.