Cloud storage is bad, I strongly advise anyone not to use this, do you know why hardware wallets prevail over everything? It's because of their offline capability, even if your cloud storage is very strong, you strengthening your recovery seed online makes it the stupidest decision still.
There are different types of malware, or let me say that there are different levels to malware, some are so sneaky that your security and anti-malware might not be able to pick them up or identify them.
It is an unnecessary risk, and it's not worth it IMO, the biggest data losses in the world happens through cloud storages, they are not as secured as they seem, Being open source is far from cloud storage companies, how will you know if you can trust the company itself and its workers? Haven't you heard about inside jobs?
Think about how many computers your data travels through when you send it to a cloud storage service. Would you really want to send data that you don't encrypt yourself over there?
Also open source has nothing to do with this - this is a network security issue. If anyone is keeping logs or is inspecting the traffic using internal SSL certificates on the company servers and your mnemonic phrase happens to be there then it's game over.