I'm actually a facilities manager by trade and am planning on DIY-ing my new flat. DIY in a sense to get my own contractors to help with the various works.
The problem is with the hacking, you'll still need to apply for a HDB hacking permit. Then there's haulage and refuge because you can't just chuck the debris anywhere.
Other than that, doing your own electrical, masonry, carpentry works shouldn't be an issue.
Lastly is the timing, it will be very tempting to just complete a certain task, but you'll have to remember to stop the noisy wet/hot works by 6 pm, if not your neighbours will complain and you'll get into trouble with HDB.