The Sony RX0 II is much better in terms of overheating than the original RX0 (which was terrible), but it's still capable of overheating in longer duration record especially in warmer environments. These are the recording times Sony says can be expected in the RX0 II manual:
In terms of image quality, the RX0 is quite a bit better than a GoPro or other action cameras, likely due to the larger 1-inch sensor in the RX0. You also have way more control over the image in the RX0 since you have customizable picture profiles, custom white balance settings, S-Log2, a lot of the things you might find in a larger Sony camera or camcorder.
The lens on the RX0 is a 24mm full-frame equivalent, which isn't nearly as wide an angle of view as the more fisheye lens on a GoPro, which could be good or bad, depending on what you want to capture. The image stabilization is better in the more recent GoPros and the GoPros also have better high frame rate modes for slow motion capture, including 4K 60, 2.7K 120, and 1080p 240, whereas the RX0 can only do 4K 30 and 1080p 120 (although it does have some 2-second and 4-second duration higher frame rate burst modes like on the RX100).
Ultimately, this means the RX0 is less suitable for typical action camera applications where the higher frame rates, wider field of view, and better stabilization of a GoPro are often desirable. Which may explain why the RX0 hasn't been a huge seller. But for other situations where you don't really need an action camera, but still need to use a very small camera, then the RX0 can often deliver a better image.
In terms of reliability though there's not really a clear advantage since the RX0 II can still overheat when recording internally. And in terms of recording time on a single battery, the RX0 is actually a bit worse than a GoPro.
