LibreSSL aims to be a mostly API compatible replacement for OpenSSL, BoringSSL does not. Literally the first text on the BoringSSL web page.

    BoringSSL is a fork of OpenSSL that is designed to meet Google's needs.
    
    Although BoringSSL is an open source project, it is not intended for general
    use, as OpenSSL is. We don't recommend that third parties depend upon it. Doing
    so is likely to be frustrating because there are no guarantees of API or ABI
    stability.