Rats, chimps, pigs, rabbits, mice, etc. are not human and do not react the same way as humans do to the same product. Even different species of animals don't react the same. Fact: putting dog flea drops/collar on a cat will cause incredible discomfort to the cat.
I don't think it's unethical to test products/nuclear weapons on humans, as long as they agree to it. Animals cannot offer their consent to have potentially hazardous substances and situations imposed upon them, so why do we do it? Animal cruelty is just as unethical (more so in my opinion) as cruelty to your fellow humans.
The supreme advantage to human testing is that the human can tell you exactly what it is thinking and feeling during and after the testing. Your average human could easily adapt to laboratory life. *cue misty lab sequence*
The scene: a spiffy, high-tech lab containing many expensive machines and a separate room for the human test subjects. Three test subjects are lazing on a couch eating People Pellets for optimal health.
lab tech 1: Man, this is so much more efficient and conducive to our research. Why didn't we test on humans before?
lab tech 2: Because early scientists saw animals as a lower lifeform that deserved no respect. It's now widley acknowledged that humans are very low creatures. And they'll do anything for money.
lab tech 1: Heck, if they need money bad enough, they're just happy to get a roof over their heads and a steady supply of nutritious food! Those losers in there *motions* cost us only in upkeep!
test subject 1: Hey, this isn't soundproof, you know.
lab tech 2: Shut up in there, you.
test subject 2: *to test subject 1* He's right, though, we needed a place to stay. I don't know about you, but it's tough being a bum.
test subject 3: I can't sit on this couch all day long. *get up and runs on giant wheel* Wheeeeeeeeeeee
*end misty lab sequence*
Don't you see? Human test subjects are the most efficient way to go about it! Guaranteed living quarters, constant high-quality food, free health care, interaction with other people. Life as a LaBrat has it all!