Biggest advantage to being a Taiwan national is that you can live, work and retire in Mainland China with relative ease if you so desire to. For any other nationality, it’s a pain in the ass to get a work visa, and often impossible to retire in the country legally.
Second biggest advantage is not being treated like a foreigner on paper when opening bank accounts, getting mortgages and applying for special deals that usually don’t include foreigners. However, when dealing with real people, there will obviously still be some discrimination depending on your ethnicity. I’m going through the process right now, and am looking forward to getting a mortgage and taking vacations to the mainland without stopping in Hong Kong on the way to apply for a tourist visa.
I can’t speak for the process of naturalisation from a plumb blossom APRC, but from a marriage ARC, you do still need to renounce your original citizenship (but only after your application to naturalise has been approved). Some nationalities, like British and Australian can regain their original citizenship back after renunciation by paying a small fee, meaning they can have both Taiwan and British or Australian nationality at the same time.