I've never run into Bluetooth congestion problems on public transit or busy streets. BT has a lot of problems, this really isn't one of them.
There are some BT devices that have poor antenna designs and underpowered transcievers, where the proximity of other humans — or even your own body — can significantly attenuate the signal. But this isn't a problem with the spec itself, and there's plenty of devices that have shown it's possible to do it right.
BT runs into congestion issues quite often in my experience. I’ve experienced connectivity issues in areas where there are a lot of wireless networks.
Considering that WiFi and Bluetooth both run at near 2.4GHz - I’d assume that areas with a lot of wireless networks could (and have in my experience) interfere with BT.
This wasn’t limited to 1 set of devices though. Tried it with my AirPods and a pair of NC Sennheiser headphones. Both cause issues when walking into busy (in terms of WiFi) areas.
There are some BT devices that have poor antenna designs and underpowered transcievers, where the proximity of other humans — or even your own body — can significantly attenuate the signal. But this isn't a problem with the spec itself, and there's plenty of devices that have shown it's possible to do it right.