Don't confuse new/old technology, and a low/high mileage bikes.
New/old technology - The more complicated you make something and the less 'mature' the technology, generally the more problems you have. So on a technology comparison, since say the mid 1980's no difference in reliability. A battery failure is just as likely to happen regardless of the technology. Age and lack of maintenance will decide its fate. If you did have to pick one, I'd say the simpler mature technology is marginally more reliable.
High/low mileage. Is there any dispute that a low mileage well maintained bike of any technology is likely to more reliable that a high mileage poorly maintained bike? A higher mileage well maintained bike should perform without any problem.
If there has been a technology improvement that has made a significant difference in terms of reliability to bikes from say the mid 1980's like to know what it was, as I can't immediately think of one. Not performance, reliability.