So, what's so bad about engine retarder brakes?

Factual Questions
1

The Beloved and I are just back from a car holiday, and we noticed that in a lot of towns, as you came in, there was a sign like: “Please Do Not Use Engine Retarder Brakes” or “Use of Engine Retarder Brakes Prohibited in City Limits.”

So, a two-part question: what are engine retarder brakes, and what’s so bad about them?

2

IANAT(trucker) but I think it might be those noisy compression brakes. It may be something about noise pollution.

3

Straight Dope Science Advisory Board to the rescue!

What are “jake brakes” and why are they prohibited in some locations?