The concept of being able to just land on another planet and start chewing on the inhabitants without an immune system customized to the local bacteria is kind of laughable, yes.
There is plenty of great sci-fi out there that assumes alien civilizations will be of benevolent character, but the big issue is the tech gap which would make it so easy for them to abuse us, no matter how good their intentions. Any civilization that a) finds us and b) can get to us across interstellar distances is automatically in the realm of magical sky-gods in comparison to us. If that is the case, it's almost impossible for a relationship with them not to be exploitative of us, simply because there is nothing we can offer in return. Anything we have that they want (our water, the meat on our bones), they can just take. They wouldn't even need to conquer us or even make their presence known to us, most likely.