I think an ABC's career prospects in Taiwan mainly comes down to language skills and culture know-how. Not so much where you were born and what passport you hold.
But I also believe that if employed for a job where English is a major factor (i.e. teaching English) and you're an ABC that is not fluent in Chinese, I think you'd be disadvantaged when compared to a Caucasian who also does not speak Chinese. It has to do with credibility. If you're having Japanese food, you'd prefer that the chef was Japanese. And for a Swedish massage...well, you get the idea.
If I was an ABC who was fluent in both Chinese and English, I'd head for one of the large exporting electronics companies. I got a couple of ABCs in my company working in sales and product management jobs, and they all have a major advantage over locals simply because they can communicate things clearer with regional offices and customers.
Taiwanese often use email instead of the phone as they can easier read and write than speak and listen. If they were to use the phone, they'd often have to ask the person on the other end to repeat themselves several times to make sure they understood, but being Taiwanese, they are afraid to lose face, and won't ask. Especially if a more senior manager is present. I got a friend of mine with crap English who used to record his weekly conference calls with the US office. On the way home he would listen over and over again to these conference calls to figure out what he had actually been listening to, and in some cases, what he actually had agreed to!
But what all this over-reliance on email communication leads to is that things take a lot of time. I see some locals that spend almost a whole day just to put together an email to explain an issue, but when they get a response back the next day, the regional office or customer still don't understand what they meant. I've often seen issues where it took 4~5 emails back and forward (and 3~4 days because of time zone delays), before the issue was resolved. An ABC can pick up the phone and clear up a similarly complex issue in 10 minutes or less.
For the culture part... it just takes time. I do believe the electronics companies have a less conservative culture than the other and more traditional industries though.