Pretty smooth once you respect the limits: 500 calls/min + 10 concurrent per realm. We run a per-realm token bucket and queue work; If you throttle and batch, you won’t hit 429s, but I talked to a few QB app owners and bigger apps tend to find it restrictive.
I know, it use to be that being an eco system partner was something companies wanted as it solved the issues for their customers, now they use it as another revenue source.