SQL is not limited, InfluxQL is. And, flux is incredibly verbose and error prone (at least a year ago when we last looked at it). However, with the transformations in grafana, there's no reason with flux, in the majority of cases.
influxql is limited, yes, but much less verbose for time-based aggregation than the equivalent sql queries, which is why I like using it for adhoc visualization and inspectibility. (I don't like flux). to me it's great for a limited purpose. writing the equivalent sql queries would drive me up the wall.
For sure, we were evaluating timescaledb and influx, we quickly came to the conclusion that while a programmer could write SQL queries to do what influxql lets you do, they're very complicated compared to the builtin UI that even non tech people can understand.