Windows MR (both VR headsets and the HoloLens), Magic Leap, Vive Focus, Pico Neo, and the upcoming Linx all use inside-out cameras, all with their own implementations. HTC Vive, Vive Pro, Vive Cosmos, Valve Index, Varjo XR-3 and VR-3, and PiMax headsets are the only ones using outside-in tracking anymore, and they're all using specifically Lighthouse.
First of all, you just don't really do that very often. People have rotator cuffs and elbows that make any action in those regions fairly uncomfortable.
Second, the all current VR systems primarily use inertial tracking. The visual tracking is only there to correct for drift out of the reference frame. Whether it's Lighthouse or Rift CV1 outside-in cameras or inside-out cameras on every other system, you can put your hands in the sensor blind spot for several seconds before it becomes a problem.
99% of the time, you're working with your hands in front of you. Lighthouse doesn't care about your hands in relation to your body. But it does care about your body in relation to the base stations. Lighthouse's blind spots are constantly changing over the course of your play session. Quest's are always in the same spot.
So many times I've found myself in a corner on the opposite axis of my base stations and my own body is blocking my controllers' view of the base stations. When that happens, you have to have enough awareness of what is going on to understand why your hands start slowly floating away while you are trying to work on something, having forgotten your orientation in the real world room. It's literally immersion breaking.
"Inside-out cameras can't track behind your head" is really not the problem that your random Valve fanboy on Reddit makes it out to be.
Controllers behind the head are tracked by some algorithm magic that fuses the last seen position by the cameras with accelerometer and gyro data for the blind spots. Seems to work like a charm. Probably not as good as full lighthouse system, but good enough.
Every extant tracking system uses IMUs as the primary tracking sytem. The Lighthouse base stations and the Oculus camera tracking are used to correct for sensor drift.
You need it to be this way, because the IMUs can run at fairly high frequencies (200 - 1000Hz), which is (in part) keeps latency low. The data paths and processing needed for the reference frame corrections are so complex that they can't be run anywhere near as fast. It's why the hand tracking on the Quest is so high-latency: there's no IMU on your hand.
And it's not "algorithm magic". It's mostly just Kalman Filtering.