Most common on low step bikes, this battery's location advantages include easy access to remove and reinstall the battery - instead of having to stoop down. Secure - a common problem with eBike batteries is that when they are removed, folks can forget to load it back in correctly (you need to listen for the "click") which can lead to having your battery dislodge and start tumbling down the road. Since replacement batteries can be $800 and up, this can be a costly mistake.
Challenges on older designs include:
Swimming frame a battery too far back (on a frame not stiffened in the rear), and the bike can "swim" when pedaling out of the saddle.
Top Heavy - likewise the battery higher up can have a similar effect.
Lighter weight batteries on more modern eBikes can minimize most of these effects