I'm not sure if attiny+ftdi really makes sense anymore these days. Checking digikey for prices (single units), that attiny84 costs $1.7 and the ftdi chip is $2, combined $3.7. Meanwhile you can get USB equipped PICs for $1.5 and ARM chips from Freescale and NXP starting from $3.
Of course there is the complexity argument that goes both ways; with attiny+ftdi you need to fiddle with soft-uart and need to design with two chips (with supporting circuitry) instead of one. With ARM you are dealing with more complex MCU and USB is not abstracted away in some blackbox ftdi chip.
Hello, author here. As I said, mostly stressed in the first post, I'm doing things the hard way as a learning exercise, building a bunch of infrastructure myself. I would use a better radio transceiver and a chip with actual USB and UARTs, etc. if it wasn't a self-study project. :)
Of course there is the complexity argument that goes both ways; with attiny+ftdi you need to fiddle with soft-uart and need to design with two chips (with supporting circuitry) instead of one. With ARM you are dealing with more complex MCU and USB is not abstracted away in some blackbox ftdi chip.