Bottom line: wallets use three incompatible ways to turn a BIP39 phrase into a Monero wallet. Two wallets, Edge and Stack, do not use BIP39 for Monero at all. If you restore a 12- or 24-word phrase in a wallet from another group, you get an unrelated, empty Monero wallet, and nothing warns you. None of the schemes is weaker than the others; the problem is compatibility, not security.
Every BIP39 scheme ends at a normal Monero private spend key s, with view key