I can vouch for a strong relationship between Hapkido and Daito Ryu. Almost everything I learned when I was training in Hapkido (with Master Kim Sung Do in Sydney in the 80s) I've also seen in Daito Ryu. What I was taught as defenses against rear grabs is virtually identical to the Ushiro set from the Daito Ryu Hiden Mokuroku (as taught by Kondo and Kato).
There are additional techniques in Hapkido - most notably striking and kicking, but also techniques that appear to have been borrowed form Judo.
There is also internal training in Hapkido - although this seems to be very poorly understood by most practitioners (you could say the same for Aikido or Karate though).
Much of Hapkido that I have seen recently seems to be taught very poorly - as a sort of add-on to TKD. This can also give a negative impression, but we must not confuse the art with some poor examples or some low level exponents.