multiplexers are fine if you're either doing slow signals or uniform outputs. They're not worth the hassle if you want multiple high-speed connections.
Not only that, also a full bloated OS wasting CPU cycles.
RPI straddles the gap between regular computers and embedded systems. It can perform some of the duties of each, but the drawbacks become clearer the further away from the middle ground your use case gets.
I can see why the Raspberry might not be a good replacement for Teensy - power hungry (ESP32/STM32 have fantastic ULP/Deep sleep) and/or form factor.
> pin count?
maybe but there are ultra cheap analog and digital multiplexers available.