That's my go-to reference when looking for new routes. We can really think of its reliability as a signal-to-noise ratio: inexperienced road cyclists might cause the map to show significant traffic on bad roads while experienced cyclists choose good roads. There are presumably fewer cyclists entering the realm of road cycling than there are experienced road cyclists. The new road cyclists are likely aware of the dangers of the sport, so in most cases, they probably seek resources for planning rides (e.g., cycling groups).

That brings up another point: Group rides will generally use good roads, and there will be multiple cyclists simultaneously contributing to the same route on the heat map. The boldest paths on the heat map may then very possibly be routes ridden by group rides, suggesting they're very safe.