On highly concurrent PG systems by when using parallel queries you are sacrificing throughput for better latency. You really don't want to use more than a fairly small number of workers per single select.
Not a problem at all if you provision your server properly so you aren't IOPS bound, though there are plenty of databases which simply won't fit on NVMe drives, so in those cases, yes it could still be a problem.
Not likely to be a problem if you have FusionIO drives. Likely to be a problem sooner or later on everything else. A definitive recurrent issues on all cloud providers and NAS/network drives.
