I view these things as complimentary ... each having its pros and cons, and uses.
Axoloti has the benefits of a proper RTOS, a dedicated device, instant startup , completely open source, great community .. and a patcher that supports all of this for all levels of user.
PI/Bela both have Linux and PD, sure you can argue the efficiency, but this is not always needed... and for example full usb support, and libusb is a big reason for me personally. ( I can't run my Push 2 on axoloti, even if I want too ) ... it cant replace axoloti, but its a great alternative to a laptop to complement it.
Heavy, yeah... not open source, but i use many commercial products, open source is not always a 'deal breaker' for me - perhaps heavy is useful, perhaps not... perhaps useful for some things and not others.
but Bela is NOT tied to PD/Heavy ... its got its own C interface, and even IDE, sure only useful if your a C/C++ developer, so OK for me.
Alsa, Bela true.. if you want his, get a PI ... but I think the point of Bela, is a direct low level interface, like axoloti an api close to the hardware, for efficiency. I don't think the difference to Axoloti 1.2 ms is massive, but the 20ms of the PI is significant.
Bela - usb port, yeah but supports a powered hub , and also ethernet... both very useful for me.
(i.e. my axolotl's will all be hosted by by it... and the usb hub will also power the BBB)
a great time for mobile/portable music making, many choices and cheap enough you can have a collection, and use the best of each of them.