I'm planning to go on a long trip around the country where speed limits are ridiculously low, even on highways, and that's all the road police care about (even the slightest of speed-limit infringements) and was wondering if a radar/speeding-camera detector would be worth it.

Can't use Waze or anything like that because these apps don't have enough data on Southeast Asia (Vietnam specifically).

If they are at all reliable, how much should I expect to spend on the one that wouldn't get me in trouble?

Do they work as well at night as during the day?

And is there anything else I should know about them?