If you already know how to work on a bike and you are certain that the frame's geometry can handle the specific components you picked out, it's worth doing for modularity's sake. You won't save much money like you would have a few years ago, but you can upgrade the battery five years from now when they're much better and yours is starting to go out. Apart from fitting the components you really need to consider if that frame is strong enough. Ebikes experience a lot more stress from weight, speed, and vibration. Cheap components and ones made for manual bikes will be much more likely to fail, and at ebike speeds that's potentially fatal.

Conversions can be problematic for a number of reasons. E-bikes put a lot more torque on the wheels, both when accelerating and braking. For this reason, most E-bikes use thru-axles and disc brakes. Older bikes with rim brakes and ordinary drop-outs on the fork are much more likely to have the front wheel break free from repeated torque.

That said, if you actually maintain your bike, it will still likely be safer than many E-bikes on the road. Just keep the speed / power reasonable for the type of hardware on your bike. A 90s mountain bike shouldn't be going above 20mph. A DIY E-bike on an old frame is not going to replace a moped, let alone a car - but it will get you around a lot faster than walking.

Like the others said it's rad (if you have the skills and the motivation), but not practical.