I commuted LGA to IAD for 5 years. 90% of the time I flew to DCA then train and bus to IAD. Way more reliable then the IAD to LGA UAX flying. Was most definitely annoying, but super reliable.

From DCA there is an airport exit direct to the metro station a couple hundred meters away. You take the metro a couple stops and connect to I think the Yellow line (I forget now, but you'd see it there). Take that to the end where a bus leaves every 15 minutes direct to IAD. I also believe there is a metro line under construction that will eventually go direct to IAD.

Anyways, like I said LGA-DCA on Delta or AA then train to bus was, for me, WAY more reliable (and had hourly service) than LGA - IAD on UAX.