The steps in the genomic analysis pipelines are not always embarrassingly parallel. The degree of parallelization cannot be increased to an arbitary number. In addition, the parallel executions can give slightly different results when compared with the serial output, so we need "safe" data-partitioning schemes and rigorous error control.

If you are further interested in parallelization schemes for genomic pipelines, please have a look at our paper on the strengths and limitations of big data technology for genomic analysis (published last week) - https://people.cs.umass.edu/~aroy/sigmod17-roy.pdf