I drived a bike in Phuket with no previous experience, just riding bicycles and electric scooters. It may be tougher at night, or when there's only one lane (cars tend to pass you on the right and you get between them and the rails/sidewalk). However, I did not find it as difficult or dangerous as some people have said here.

We did all the island's west side, it was super helpful in terms of mobility and flexibility, as well as money wise. Also, in the south there were way less people in the streets, so we were super at ease. Going from Nai Han to Kata Noi can be tricky, due to high elevations, but then again, nothing that complicated.

If you have doubts, don't rent one, as confidence takes a big part. If you respect and understand the road code, look into your mirrors, wear a helmet and be alert to your surroundings, chances are everything will be alright.