Analogies and metaphors are a tough arguing tool to use and need to be utilized skillfully in order to be successful. A lot of people absolutely suck at it. The argument that I (and many I've seen) usually make against analogies is not "those are two different scenarios, so it's an invalid argument" but rather "there are two totally different operating conditions around those two scenarios, so you can't make a valid comparison."

To elaborate a bit more: Analogies and metaphors work in situations where it's hard for one party to understand your side of the argument because of technicalities, so you break it down for them. Explaining a biological process, for example, like a highway can be useful even though the two things are totally different. But arguing that traffic on a highway works like a human biological process is not a valid argument to make because there are two totally separate operating conditions around those things.

So basically, what I'm saying, is that it's fine to use a metaphor in an argument when you're explaining a process of your argument. But it's not fine to use as supporting evidence of your argument.