Electrum seeds have obvious advantages over the former and generating it from some other sources raises some security concerns as well.
There's a downside of using electrum seeds, at least if we're talking in the long term. Electrum seeds, as said by their name, can only be generated from electrum. But, BIP39 is a proposal and many wallets adopted it. Electrum doesn't propose anything, the developers simply believe that their way is the proper one.How sure can I be that electrum will exist in 10 years from now? A lot, but I can feel more sure with BIP39.
There's no nested Segwit for Electrum seeds and for which BIP39 would be your only option.
I still don't get why they haven't implemented it for nested Segwit...I believe Electrum no longer lets you create a legacy wallet through the GUI.
It allows you: