Bali in december?
 

In december it is the rainy season, but is that a problem? (roads, air, heath)

I've been to Bali in December several times. Yes, it is rainy season but it didn't interfere with my enjoyment of the place.

Suzanne: Bali has several micro-climates and it could be raining hard in one area for an hour or two and a few feet away or over the mountains it could be sunny and bright. Ubud, Amed and and Seminyak have very different overall climates.

As for road, air and health concerns, I'm not really sure what your specific fears are, but Bali doesn't normally have the kind of flooding problems that a concrete jungle like Jakarta or Bangkok has, if that is what you are worried about. Rains may fall heavily in a specific area for a while and create temporary floods, but it usually washes away in an hour or so. There are places where potholes are created, or parts of roads even, but these are usually not major problems, especially in major roads.

I can't imagine what air quality issues would be affected by rain, but then again, I'm not a doctor, nor do I play one on TV.

weather2travel.com climate guides more rain not
bad would go just stay flexible for best experience.

You will get rain in December: periods of intense downpours and overcast skies alternating with clear, sunny days. Flooding problems are minimal.

To me the biggest downside to Bali in December (especially late December) is that it's the height of the high season. Rates are inflated, restaurants are crowded, traffic is snarled, services are stretched.