Is it risky to allow SMB traffic to the Internet

I'm not sure what operating system you are using, or if/what firewalls you have implemented, but exposing the SMB service with unfiltered access from the internet is asking to be compromised.

Also, a Man in the Middle attack would be the least of your worries. Someone could easily gain root access to your computer and consequently your entire network. Famously, the ms08_067_netapi exploits the SMB service on Windows XP on port 445 using a relative path stack corruption. I could easily execute this and gain root access in less than a minute assuming I know an IP address. For any random attacker, port scans are done across the internet continuously. Someone will find your open port.

I would recommend that you close that port and find a different solution for what you're trying to accomplish.

There is a risk of exposing your credentials via broken SMB NTLMSSP_NEGOTIATE implementation, as descibed here: https://www.cybersecurity-help.cz/blog/167.html

Yes it is risky.

While I don't know about any "exploits" that may be out there in the wild, what I do know is that anybody with the right username and password could get access to the disk drives that are exposed.

Presumably there is an administrator-level account on the computer that would allow full access.

I don't believe SMB limits the speed at which someone could run a dictionary of most-used passwords to try to get into Administrator, but it is possible to do it slowly, even if it does.

All in all, it seems like a bad idea to do it without first setting up an encrypted tunnel of some kind like a VPN. Or like SCP in Linux.