It's just a pain to write and configure that many different devices each with I assume external SPI ROMs and bitstreams. Timing analysis would also apply locally, and you'd have to propagate that through yourself. Since they're each buffered at the edges, I don't think EMF is a big consideration, but timing would be a challenge. I expect routing to be the least problematic as you can just route the signals to external pins in whatever configuration you want.