I don't think it's that bad to ride Class 3s anywhere, but it is likely to induce bad behavior because of how fast the bike can go. Motorists labor under a similar mental barrier, which why they honk all the time even when you're traveling at the speed limit in a low-speed area.
So the right thing to do is to adopt a personal speed limit based on what the surrounding traffic looks like. If that's too easy, scale back the assist until it's just right. For a bike lane, I'd say that limit is about 16-17 mph, just shy of the 20 mph Class 1 limit. That's not unreasonable for very fit cyclists and ebikes have more powerful braking power. You refrain from using the top speed because the bike is more substantial than a typical racing bike, and can do more damage in a collision.
In a shared lane situation, whether or not the law mandates it, I'd say ride at 6-8 mph. This is running speed. It is extremely unlikely to injure anyone seriously, whatever happens. If you have to stop, stop. Ebikes, and particularly powerful ebikes have the advantage of effortless acceleration from stops, so might as well take advantage of it to stop whenever it's advisable.