If you don't synchronise it's not an issue :-) but if you have 100+ instructions in motion at any one time creating a bottleneck at the end of every basic block

If you don't synchronise it's not an issue :-) but if you have 100+ instructions in motion at any one time creating a bottleneck at the end of every basic block would likely have a big effect on how things run. Normally we retire up to 8 instructions per clock - merging 8 bits of sticky state is some simple logic (some or gates, it doesn't matter what order they get processed in), merging 8 saved PCs (which one do you save?) is harder, it probably means a priority encoder (order probably does matter) and a 63 bit mux (still doable in a clock though)