You have a point there.A lot of bikes will have carried heavier loads than a battery & motor via commuting or touring - although admittedly not on their bottle cage mounts so worth keeping an eye out for cracks on the down tube.
Personally, i wouldn't recomend someone to convert an ordinary bike to an e-bike, especially an unrestricted one, which is usually what people think of doing. I think it is better to have an over dimensioned frame rather than an under dimensioned one. An e-bike weighs approxamatly 10kgs more than a normal bike without assistence. With the weight of the rider and some added luggage, then you could easily be over the 120kgs manufacturers safety limit. The safety aspect is what should be given the largest proportion of consideration, in my world anyway.
A bit of fun could end up being a nasty wake up call.