Well it depends on the ebike. Compared to most ebikes on this forum, my ebike is very mild, an Ezee Torq. A "turn-key," "frock motor," pay up front solution. It has some limitations to stay within the legal requirements. But at least as far as I can tell, it is very rain resistant. I have a relatively short commute, about 5 miles, one way. I purchased the Torq in late August and since then have put on over 1,100 miles commuting to work many days. So that's a fair number of miles my car hasn't been on the road. Like LI-ghtcycle, I am in the Portland OR area, so there are lots of rainy days in the winter. The bike has never failed me, no problems with water. I have gone through deep puddles at some speed so that water splashed up and filled my shoes with water. I learned that I needed higher boots to keep out the water. But the Torq never faltered. Despite rain pants, water resistant boots, a rain cover for bike helmet and a good rain jacket, I have come home soaked to the skin. The bike keeps on going. For various reasons I have taken the car occasionally and on a few occasions these have been because of the weather. However, for the most part this is because of job requirements or I wimped out on very cold (much below freezing weather) or very wet weather, not because I thought the bike might fail. My bike is a commuter, not a recreational vehicle, so I depend on it to take me through the rain. On the other hand, I know from problems with water leaks in my home, that water is insidious and eventually will get into any weakness. My Ezee Torq ebike seems to be relatively resistant, but I don't think that anything is immune to the damaging effects of water.
Rich