I don't think this is even just an AI denoiser problem. Rain effectively looks like haze for the parts of the scene that are at sufficiently long distances. Therefore it's difficult to use dehaze with rainy scenes. The harder the rain is falling, the more limited your ability to dehaze. It's much the same with denoise. You won't be denoising at 100% in Topaz if it's dark and raining hard.

One day smartphone cameras might get there, but right now, the sensor technology isn't there yet. The problem isn't merely noise. It's that rain and snow are moving about in the scene, which means that the camera can't just do its usual trick of taking multiple exposures.

AI denoisers are nice, but they aren't strictly necessary for ILCs. Even those full-frame cameras without IBIS are able to take pictures of nighttime snow just fine.