Fair point. But they are far easier to define and have a clear concept of orthodoxy/heterodoxy, which most polytheisms don't. A Roman pagan didn't really have a concept of heresy, there was just the way Romans did things (obviously, to them, the correct one) and the way others did things (the wrong way, for the Romans), but there were no "correct" gods, just the gods in a particular area. Obviously gods like Hapi, the god of the Nile flood, were not portable and had no particular conflict with non-Egyptian paganisms, while gods like Anubis were syncretised with roughly equivalent gods like Hermes/Mercury. The idea that in Rome you'd pray to Iupiter for rain, but if you travelled to Greece you'd pray to Zeus, and if you traveled to Syria you'd pray to Ba'al Hamon, wasn't really a difficult one, especially if you weren't a philosopher. Some gods, like Isis, travelled well and slotted well enough into other pantheons. This was hardly the first time the Romans had syncretised their religion, the classical Roman pantheon was composed of Etruscan, Greek and native Latin deities.