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Is it secure? Any risk of losing cards?

No risk of losing cards. But I don't recommend using it on any account which you actively use for posting/voting. Because you give them your posting key, and basically, that's the most damage a posting key can do, if misused: posting or voting with your account.

No financial transactions. No card or other Splinterlands asset transactions.

PS: After HF26, I also don't recommend having HP in such accounts, to avoid any temptations. Only RCs.