Given the benchmarks:

Small static file (174 B) - the bread and butter of static sites:

server req/s p99

zeroserve 36,681 5.4 ms

nginx 31,226 7.8 ms

Caddy 12,830 22 ms

zeroserve serves small files about 17% faster than nginx on a single core, with a tighter tail. HTML pages, small JSON, CSS - this is the case zeroserve is tuned for.

Large static file (100 KB):

server req/s throughput p99

zeroserve 8,000 782 MB/s 22 ms

nginx 7,600 773 MB/s 28 ms

Caddy 6,084 590 MB/s 44 ms

I'd go with a more storied project that's been audited, battle tested, hardened etc than this upstart. There's not enough improvement to justify the risk.