Can I open wallets created with other BTC clients with Electrum too?
You can, but the problem is if you import its master keys to other wallets or BTC clients, you are exposing it to an online machine, and having multiple clients with a single seed can compromise your wallet in anyway because of too much exposure. That's too risky. It would be better if you want to use Electrum, then just generate your wallet in Electrum, or if you are using other Bitcoin wallet/clients then generate your wallet on that wallet to avoid more exposure.
What If a user wants to migrate from one BTC GUI client to another?
For me, for safety and to avoid much exposure, instead of importing the backup seed to another wallet, then sweeping would be the best option, or sending all your funds to the new wallet in another wallet that you want.