Just for S&Gs, let's assume, for sake of argument, that we have somehow magically converted these participles to prepositions. Let's take the sentence Considering the hour, it is surprising that he arrived at all.

Why is it bad to treat Considering the hour as a participial phrase? The standard answer is that it is a dangling participle, i.e., it is not modifying anything in the sentence. What do we gain by thinking of considering as a preposition? Not much; prepositional phrases need something to modify, too. We don't say things like: Into the house, I saw a cat. All we've done is change a dangling participle into a dangling preposition.