Is there any truth to the claim that it was due to 'similarities' between Arianism and their ancestral beliefs, and if so, what similarities were there?
I asked a similar question a while ago. Here is the post, see if the answers might be of any use.
I'm sorry I am not exactly answering your question! (Getting you all excited by a messages notification.)
I have been intrigued myself about this. Constantius and other members of Constantine's family were known to be Arians as well.
Arianism in these frontier areas seems to be more legitimate than in the Bosphorus and Byzantium. In these areas, while there was actual adherence there was a lot of shit slinging (not literally like the Stylites). Athanasius for example would label Arians like McCarthy label communists. The adherents in other parts of the Empire did not have this issue as much as in and around Constantinople.
However there was a lot of evangelisation by Arian Christians in the barbarian frontier regions. Michael Frassetto discusses this in his Encylopedia of Barbarian Europe.
Beyond this I guess your question has encouraged me to have a look! This, above, seems to be all I know for the moment.