I have projects that can equally use either a Teensy or a few different Feather boards, and frankly the Feather boards are always the prefferred option beacause:

* assymetrical footprint - the assymmetrical dip footprint means the board has polarity protection. I have hat/receiver boards with zig-zag stagger dip rows that work as free componentless sockets to receive a feather board with pins soldered. The Feather versions are far more convenient and safe since they can't be plugged in any way but the correct way.

* The Feather standard being a standard - ecosystem of compatible boards with compatible footprint & pinout at least for main functions.

* lipo charging circuit and jst plug, the automatic ups functionality, usb charging, removable/replaceable via standard plug

* off-board power on/off - I very much make use of the trick Feather has where a hat can turn the mcu board on/off without being the source of power to the mcu board.

* Perhaps a small detail but Feathers with sd card slots have the card sense pin wired up to a gpio pin while the teensy's do not. On Feather I can have a hardware interrupt fire when a card is inserted or ejected, and I can't do that on Teensy.

* keeping all parts on the top surface - there are other mcu dev boards with some similar functions but they are less convenient to use as a module in a project since they may have buttons, jst plug, sd card slot, or other stuff on both sides of the board, which makes them inaccessible when mounted onto some other pcb.

Teensy has a few points in their favor too but I really value these facets of most Feather boards.