In Ethereum, clients must monitor for the successful progpagation of transactions (each with a nonce) in order to broadcast a new transaction (with a later nonce).
If, due to low gas or some other reason, the earlier transaction is not sufficiently propagated, the client must issue a replacement transaction with that nonce (and ensure it propagates) before a transaction with a later nonce will be accepted.
This limits the utility of pre-signed and offline signed transactions.
Polkadot has taken some steps towards more flexible transaction validity but must transactions still be ordered primarily by nonce?
Another way to phrase this question would be: can a later-nonced transaction ever take priority over a lower-nonced, but perhaps faulty or non-existent transaction?