MapReduce was used at Google for highly inappropriate things. For example, the machine learning system I worked on, Sibyl https://www.datanami.com/2014/07/17/inside-sibyl-googles-mas... was implemented using mapreduce but there was no real technical justification for that- it's just that there was no other system that could scale to the volumes required or handle the constant failures endemic to GOogle's internal systems. It ended up requiring all sorts of heroic work to make MR scale, for example map-side combiners (which "reduced" items with common keys in the map output before it gets flushed to the shuffle files). All of this got replaced with TensorFlow and only the good bits of Sibyl were extracted to TFX.