Batteries that are integrated into the downtube are increasingly being used to keep an eBike looking stealthy so they blend in better. Like downtube batteries, this battery's location advantages include better handing because the bike will handle most naturally the closer the battery is to the cranks where the pedals are. This location can also be used on step through designs - allowing easier entry at the same time having ideal weight distribution.
Bosch was a little late to the party, but is the belle of the ball with the PowerTube 500. You will even see dual versions on some Riese & Müller bikes. They addressed a big concern with some integrated batteries - when unlocked, it pops out almost an inch from the mount, but does not release completely until you grab the battery and release the catch, which prevents the battery from falling out.
A common problem with integrated batteries is that they can pop out if not properly clicked into the frame mount. Replacements can cost $800+ so seeing your battery tumbling down the road can be a heartbreaker. Make sure to click it!
There are tools online for you to be able to estimate the range of any given battery setup, Bosch’s range calculator - select dual or single, other factors to compare. Specialized’s range calculator - select the model and other factors