That's the issue, though: it's unclear how many attacks you can make using multiple attacks. Most people are interpreting the rule using 5e as the basis of RAI, and I don't think that's really a viable solution. Xenon bases it on how many weapons you have, and honestly that makes even less sense to me, since it's easier to attack twice with the same weapon than it is to attack once with two weapons.
I don't think there's enough to support firing mode having an effect on multi-attack, RAW. Some people do. If the rule is unclear, then shouldn't there be some sort of clarification? I'd even settle for a RAI from the developer even if it's not a point of errata.
It's true that abusive cases are rare, but the rule is still nonstandard for non-abusive cases. I don't think anyone can see attacking twice with the same weapon as abusive, but it might not be supported RAW.