Techmoan is great fun to watch, but only a few of his reviews are relevant to bicycling -- the cameras he tests on his motorcycle. Some of his recommended video cameras are not weatherproof and housings may not be readily available. Or they may require cable hookups that compromise weathertight integrity.

As you noted, many Euro license plates are huge compared with US plates. So I look at details in signs and other small lettering to evaluate resolution.

A few things I've learned by experimenting with my various digital still cameras with video options:

• For traffic documentation, wider isn't better. I'd prefer a lens angle of view equivalent to a 28-35mm lens (35mm film/full frame), or 24mm at the widest. The near-fisheye view used by Ion and similar action cams isn't ideal for traffic documentation.
• Faster frame rates give you a better shot at snagging a blur-free frame. I mostly use the 60 fps option. Unfortunately that's limited to 720p on my Ion Speed Pro.
• Most affordable action and traffic cams offer decent daylight performance. Where they fall apart is in nighttime use. Three factors hinder good resolution of license plates or detailed enough views of a vehicle to identify it without a license plate: high ISO nighttime noise -- if the luminance and chromatic noise don't obscure fine detail, the noise reduction smoothing will; the slower shutter speeds often make it impossible to record blur free frames; poor dynamic range with blown out highlights and featureless shadows.
• Bigger is better. Hopefully the next generation of traffic/action cams will use the one-inch or CX sensor (in Nikon parlance), already used by Sony, Nikon and some high Samsung smart phones.
• Get a camera that can run almost indefinitely via an external USB battery connection. The Ion Speed Pro will run continuously (with each video segment being approx 40 minutes long and nearly seamless between segments) as long as the media card and battery hold out. Sometimes I run it off an external Jackery battery via a USB cord in good weather. Kind of a hassle on my helmet, but it works as long as the battery pack is small enough.

I ride a lot at night and seldom get videos clear enough to record complete license plates. So I have to rely on other characteristics of vehicles, and a partial license plate. So far the only time that's been relevant was when I reported a Domino's driver to the corporate safety office. They didn't need the video and said my description of the vehicle, partial license plate and time/location were enough to go on.

To be serious about this I'd really need the best low light performance available, regardless of weather-proofing features. So I'm considering some cameras with better nighttime performance and buying a separate weatherproof housing. Price is an obstacle for me and I can't afford the best performing weatherproof cameras.