Whats the advantage/disadvantage?

I generally use leastconn with haproxy (obvious why this isn't ideal, but it was better than pure RR) as some our posts either take 10ms or 5500ms

Less spiky workload, especially noticeable if the server weight is high, e.g. we had weights like 100 and 137 on servers since two-digit weights weren't giving sufficient precision to achieve equal load between different hardware generations.

Since the change reduces the peak load you get lower 99% latencies and thus need fewer machines to achieve the same 99% latency on servers