Last night, we saw a pretty backed up mempool. We've seen it before and we will see it again.
I had a friend who was trying to move coins to a wallet, using a mixer as an intermediary. The mixer was Helix and it has a 15 hour wallet life. (By comparison, I've used BitMixer.io before and that has a 24 hour wallet life.)
But, the mempool started filling up. After 4 hours, the transaction was still unconfirmed. I tried double-spending back to their wallet with a higher fee (to effectively cancel the first transaction) but Electrum gave me a 258 (double spend) error. I learned about RBF in my search, but it is disabled by default in Electrum it would seem.
It seemed like I didn't have any reasonable options at my disposal other than to wait it out and hope the transaction goes through. Fortunately, it did, but this really showed me a glaring danger – correct me if I'm wrong, but had that wallets life on the mixing server expired, the transaction still would have gone through eventually (as the address if valid) but the coins would have been lost as the server would have destroyed the private key.
RBF certainly can help resolve this issue, but it is NOT user friendly to run the risk of coins being lost.
While my friend did figure out Bitcoin and mixer alone, they are not a Bitcoin-savvy individual (kudos for figuring it out though).