eBike technology explained

Hey everyone!

I've been diving deeper into custom eBike builds lately, specifically trying to understand the nuances of different motor types and battery chemistries. It's fascinating how much technology goes into these machines, especially when you're piecing it together yourself. For those of you who've built your own, or even just upgraded components, what's one piece of eBike tech you found surprisingly complex or incredibly simple to integrate into your custom setup? I'm particularly curious about mid-drive vs. hub motor controllers and their programming.