The key to using Hwachas is to either use them defensively or "safely" until they've gained indirect fire or range promotions, and after that you can use them even more freely. Very soon after that they'll get the volley promotion against cities. And once you upgrade them to cannons, they get even stronger because they get the siege bonus against cities like all siege units. With logistics you'll also generate XP much faster, leading to more/faster GGs, enabling you to citadel your enemies and make more room for your Hwachas & units in general. I think they're one of the by far most broken things in VP.
I'll try a playthrough test as Korea - start with just 1 city, as authority (I'll delete the imperium settler), no killing units etc. until Hwachas come online, no building Seowons or using specialists (except for the guilds). See how fast I can catch up and surge ahead.