I know a few people who are dual citizens who don't enter Taiwan with TW ROC passport. There is plenty of downside since you don't exist and can't do many things (such as working). Until now, you also can't use eGate, not to mention the stay limit.

Unlike the US, it does NOT seem there are rules that you MUST use your TW ROC passport. However, entering Taiwan using a foreign passport does not exempt one from military duty. If they figure out who you are, you might still be refused exit. So, it is better to get classified as an "Overseas Citizen" and follow the pretty lenient rules.

For those classified as "Overseas Citizen", you can enter/exit fine as long as you don't stay more than certain number of days continuously AND cumulatively AND you apply for exit permit (which can be done online at https://niioa.immigration.gov.tw/NIA...plyForm.action).

The current exception might be that if you never have had been in the household registration system in Taiwan. The problem is Taiwanese laws get interpreted by different government employees differently so it is better to be safe.