I would like to suggest http://www.diptera.org/ as an external taxonomic authority list for the true flies. It’s full of world authorities in different dipteran families that check and contribute with it.
Yes, this is helpful as a source. I don’t think we should give it unilateral preference over BugGuide.net, since the latter site is sometimes better maintained with respect to taxonomy.
BugGuide is only for the US and Canada, it doesn’t even cover Hawaii.
For what it’s worth for now, I went ahead and added a search link for Systema Dipterorum (http://www.diptera.org) that will show up on the right-hand side of the About tab on any Dipteran taxon page in iNaturalist (under More Info). This will at least facilitate looking up current Dipteran taxa in that reference, and provide a handy entrance for performing additional searches.
I completely ignore Systema Dipterorum when curating the Asilidae species catalog. It has numerous errors and omissions in robber flies (and probably other families too). It can be used to supplement existing taxonomic sources, but by no means should be the sole authority for Diptera.