I've definitely noticed that some videos have done very well on Instagram Reels while they flop on TikTok. I think this is at least partly due to the timing. Reels was very new, and Instagram was really pushing this new feature, so some of my Reels got tens of thousands of views, while on TikTok, the same video struggled to get 200 views. The window of opportunity on Shorts is closing, in my opinion, as it is already being rolled out in the U.S. and becoming more competitive. I definitely do cross posting whenever I can, though my focus is really on long form content, so admittedly, I don't post a lot on Reels/TikTok/Shorts.