SMB2 is a more secure form of SMB. adclient uses SMB2 to download GP and does NTLM authentication. If turning off SMB2, adclient will revert back to use SMB which is considered less secure.
What is the main difference between SMB and SMB2, are there any side effects of switching off SMB2?
The main difference is SMB2 (and now SMB3) is a more secure form of SMB. It is required for secure channel communications.
The DirectControl agent (adclient) uses it to download Group Policy and uses NTLM authentication.
The side effect of turning off SMB2 is that adclient will revert back to use SMB and as a result will disable support for SMB signing. If the Domain Controller is configured to require SMB signing, group policy updates and NTLM authentication will fail.