There's nothing wrong with OEM systems if you find one that suits you, but there's a lot of BS about DIY (sometimes spread by disingenuous individuals who own or are employed by bike shops - I think). I have a Bafang mid drive which will blow the doors off any OEM system and has been used off road for seven years with no maintenance and no repairs. On the other hand, my Haibike, which is used sparingly, needed to have an expensive switch replaced (which entailed dropping the motor).