No problem with them, but one thing I've seen that can be an issue is accounting for how you distribute the weight of the batteries in the vehicle. Newer evs are generally in a skateboard (along the bottom of the car) configuration or lengthwise, where a transmission tunnel would otherwise be. In a lot of older vehicles conversions, they just hook an electric motor to whatever transmission is already there, meaning you are generally putting the batteries where the engine would be, or in the trunk. If you are putting a lot in (usually required because they are less aerodynamic), the weight can affect how the car handles negatively. There are only a few cars that don't really seem to have that problem, and that's because there were other spaces on the car. The best one in my mind for it is the Porsche (or VW depending on how you view it) 914, and there already have been a few conversations of them (the fuel tank is right in front of the drivers feet, so you can just drop batteries there and in the back where the engine used to be).