Premade or DIY
Premade or DIY, the decision can be tough. Most people don't have any experience in assembling mechanical devices or electrical work.
Overall, DIY seems to be the way to go if you can. Many premade bikes use proprietary batteries controllers/shapes and some even use controllers that won't work without the exact same brand battery.
a DIY bike can be repaired with off the shelf bike parts and the battery/controller/motor can be replaced with pretty much any other battery/controller/motor of similar spec. In the event of damage/failure of the frame the entire electrical kit can be swapped to a new bike. This gives you incredible flexibility to repair your vehicle should any particular company shut down in the future or have excessively high prices for some parts.
You can also make sure your batteries are from a name brand battery manufacture with DIY. Most prebuilds give little to no indication on who makes their batteries or who makes their cells.
That said, it is a lot easier to just buy a product and ride it. Due to the high currents involved in e-bikes it can be dangerous to DIY if the electrical work is incorrectly done. Same with the mechnical work. We highly recommend you not convert 'department store' bikes such as Walmart, Canadian tire, etc due to their low part and assembly quality and instead stick to bicycles sold by bike shops or 2nd hand bikes.