People seem to equate his lack of completing passes downfield and propensity to check down as lacking arm strength. He's only 27 and was launching the ball 50+ yards down the field earlier in his career. I'm not buying he can still throw deep at his age and I'm unaware of any serious career altering injury he had that prevents him from throwing deep.
FWIW, I think he can throw it downfield as far as Stafford can. Stafford's arm strength really shined on his ability to throw it on a line between defenders. Goff doesn't have that kind of arm strength but he can hit a Williams behind the defense deep downfield.