Many clients should by now be compatible with SMB2, and SMB2 is generally considered to be free of the security flaws that blight SMB1.
It's probably worth pointing out that SMB2 isn't actually a specific protocol, it's (at least in the most recent versions of Samba, as provided by LibreELEC) a synonym/alias for the SMB v2.10 protocol, while "SMB3" is (currently) an alias for SMB v3.11 which is the latest version of SMB (there have been several versions of v3: SMB3_00, SMB3_02, SMB3_10).
Samba v4.0 introduced SMB2_10 support, and pre-v4.0 Samba servers that only support SMB v2.00 or SMB v2.02 may not be accessible by a LibreELEC Kodi client using "client min protocol=SMB2" as the client will start connecting with SMB2 v2.10, which will fail. In such a case it will be necessary to manually configure "client min protocol=SMB2_02" (or SMB2_00) in /storage/.kodi/.smb/user.conf.
It's also possible/likely that older versions of Samba (pre-v4.0) may use different values for the SMB2 alias, such as SMB2_00 or SMB2_02, and pre-v4.2 the SMB3 alias will not be supported at all.
What I'm saying is that the precise "meaning" of "SMB2" and "SMB3" will depend on the version of the Samba client AND the version of the Samba server, as either one could have a different understanding of what each alias means, which could result in an inability to establish a connection. You gotta love Samba, it's such a mess...
So anyway, just something to be aware of when setting SMB2/SMB3 all around, and you start getting weird results - try manually configuring a specific min/max protocol rather than using a potentially ambiguous alias.
Also, don't be surprised if Mac OS gives you problems - the Samba server provided by Apple is known to have several issues.