I considered Nextflow before begrudgingly settling on snakemake for my current project. Didn't record why... possibly because snakemake was already a known quantity and I was under time pressure or because I felt the task DAG would be difficult to specify in WDL. It's certainly the most mature of the bunch.
Nobody wants to write or debug groovy, especially scientists who are used to python. It also causes havoc on a busy SLURM scheduler with its lack of array jobs (heard this is being fixed soon).