Well, it is great for the Arduino infrastructure, it isn't as ideal for the Teensy 3.x. Don't get me wrong, it can probably be used for Teensy's, but having the Arduino shield and form factor would be somewhat awkward for Teensy's.
With Teensy 3.x, it would involve multiple voltage regulators, unless they reach the $15,000 level, since the unit would boost the power from the 3.7v lipo battery to 5v for the Arduino. Then you plug this into the VIN line on the teensy which needs to regulate the power back down to 3.3v. If you don't need 5v (i.e. no neopixels for instance), you can feed the lipo directly into the Teensy, or at least regulate it directly to 3.3v. This would avoid some of the loss of energy wasted by boosting the voltage up to 5v, and then bringing back down to 3.3v.
One of the things they do is combine the battery fuel gauge and charging circuits. Usually when I've been looking around, you either have the fuel gauge or the charger support, but not both at the same time.
