I tested on Fuchsia. Everything built and all tests passed correctly, so no problems there.
Here are three timings from builds that were run without parallel rustc enabled:
5284.35user 696.95system 3:00.15elapsed 3320%CPU (0avgtext+0avgdata 2963072maxresident)k
1704inputs+61728288outputs (173653major+94432200minor)pagefaults 0swaps
5337.72user 679.63system 3:07.12elapsed 3215%CPU (0avgtext+0avgdata 2962480maxresident)k
6217992inputs+61728224outputs (174415major+97968950minor)pagefaults 0swaps
5279.77user 705.58system 3:01.22elapsed 3302%CPU (0avgtext+0avgdata 2968028maxresident)k
14080inputs+61729144outputs (182088major+96475006minor)pagefaults 0swaps
And here are 3 runs with parallel rustc enabled:
5313.74user 689.57system 2:59.98elapsed 3335%CPU (0avgtext+0avgdata 2982788maxresident)k
1048inputs+60992440outputs (155345major+96491042minor)pagefaults 0swaps
5318.76user 692.16system 2:58.39elapsed 3369%CPU (0avgtext+0avgdata 2964200maxresident)k
1137304inputs+61014216outputs (160991major+98495705minor)pagefaults 0swaps
5330.31user 691.24system 2:57.67elapsed 3389%CPU (0avgtext+0avgdata 2965328maxresident)k
1040inputs+60995288outputs (159364major+97233184minor)pagefaults 0swaps
So it looks like we saw a small but significant improvement in build time. I suspect that enabling >4 threads would lead to further improvements.
Note that there are some non-Rust steps in those builds, so the gain might be higher percentage-wise. (Only the Rust targets were invalidated, but we have some targets that depend on everything at the very end.)
This wasn't very scientific, so should probably be taken with a grain of salt. 
On the build of specifically our third party code (built with cargo; takes around 20s; included in the above builds) I did not notice any significant change.