
It's certainly true that some ex-trained protection dogs do end up in retirement homes. I have met a few ex Police GSDs for instance.
A year ago I took on an older rescue dog who has almost certainly lived a 'working' life in that sphere in the past. Apart from a few issues with her adapting to living in a home - although I think she may have lived indoors before, but mostly forgotten it - I haven't had serious issues regarding her past life. If anything she seems socially inept, but friendly towards all people, and the worst problem we have had so far is her prey drive meaning she is totally not trustworthy around any livestock or cats. I have done a few little experiments and I am 99.9% sure she has been sleeve trained, but she will come off at the first firm word, and indeed, flinch - as if expecting a punishment like an electric shock. Getting her off a tug toy is impossible though!
Unless the dog has been owned by a truly irresponsible person that raised it to be aggressive, then any ex working dog must be taken on its own merits, and that includes the fact that any services/forces/military/security personnel train very highly for good socialisation to human beings FIRST and FOREMOST. Being good with other dogs is also important but not in all cases, as many dogs work alone with just the handler. Mine is at a loss among other dogs, and only seems to respond to entire males but I have no idea what isolation she may have had in the past... she may have been kept as a brood bitch, which would explain her delight in meeting full males, and her disdain for neuters!
If you are experienced with reactive dogs, and can cope with them [meaning, you have developed a kind of ESP, and lightning reactions, lol] then I don't see a problem with taking on this one. But please feel free to fill me in with more details either on here or by PM, I won't judge.