The command doesn't fail but does it actually flush the DNS cache, or does it flush the Directory Cache which isn't used for DNS anymore (or possibly at all, on my El Cap machine flushing succeeds but any query of the cache content or its statistics replies "Unable to get details from the cache node")
The command did flush the Directory Service cache, but DNS is no longer part of it. You can still use its query command though.
dscacheutil -q host -a name www.example.com
On the topic, easiest way to see if dscacheutil -flushcache work or not is to probably do something like:
dscacheutil -q host -a name www.example.com
# Make sure it's in the cache prior to running.
sudo dscacheutil -flushcache
dscacheutil -q host -a name www.example.com
# Notice if there's a slight delay for the lookup.
sudo killall -HUP mDNSResponder
dscacheutil -q host -a name www.example.com
# Notice if there's a slight delay for the lookup.
So I see absolutely no point here.