Using cloud storage is not a good idea. Once you've uploaded something to the cloud, you can never delete it. You have no idea how many servers around the world it will be stored on, and who will have access to those servers. You say you want to wait "some years" before spending your bitcoin. There is no way you can be absolutely sure that in the coming years there isn't some critical flaw exposed in the RAR program you used, or the algorithm it used, or the cloud storage providers security, and so on. It's much safer to write it down or engrave it in metal and store it offline.

It's not "unsafe" per se, it's just that Electrum seed phrases don't use the BIP39 standard and therefore aren't compatible with other wallets. You can read here for the reasons why: https://electrum.readthedocs.io/en/latest/seedphrase.html?highlight=bip39#motivation