Los Alamos Labs in U.S.A certainly use RPi clusters. In 2017 they had a cluster of 3,000 Rpi cores and were looking to expand to 40,000 cores. The intent was to model new models of distributed computing for their Trinity supercomputer but at a much lower cost. The infrastructure they developed to host their cluster is now available COTS as Bitscope blades and server racks.
Distributed computing is possible on much smaller clusters, and is made even easier by the fact that Wolfram provide Mathematica, free of charge on each Pi, which supports distributed computing out of the box. Other distributed models are possible including Hadoop and, my favourite for handling streaming sensor data, Apache Flink.
The new Pi 4 may be beginning to eat into the performance domain of low end desk top PCs. So a cluster of four costing around £2-300 fully specced may offer similar performance and certainly much more flexibility esp. for sensor data processing. You can't build and power a sixteen core PC cluster off a car battery and leave it outside in all weathers.