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.