When sh is a symlink to bash or dash, bash limits itself to POSIX-compliance, so it should be 100% compatible with sh?

From Difference between bash and sh : > ABck to the question: If you have /bin/sh as a link to bash, then bash will not behave the same when called as /bin/sh as it does when called as /bin/bash. When called as sh, it will limit itself to mostly POSIX-compliance plus a limited set of extensions. Does it mean that whenever I come across a shell script in Linux with a shebang to sh: #!/bin/sh, even if on that distro, bin/sh is a symlink to another shell, like dash or bash, it should be 100% compatible with the bourne shell, since it limit itself to a limited set of extension? So I could execute them in FreeBSD? Is there are exception to that? Or I should be safe to assume that it will work? So if on a distro, bin/sh is a symlink to bin/bash, and a script use #!/bin/sh and the script contain bashism, it will not run, since bash will like be in sh mode?