Is it perhaps because they usually have no military background, speak no or poor Arabic and have already given up everything, as Peter Neumann suggested in today's article in Der Spiegel? It seems that "local fighters" are increasingly less likely to blow themselves up but perhaps this is because there are other willing executors? And perhaps the fact that they are westerners, either western-educated like "Djihad John" or converts also gives some satisfaction?
The UK and Germany in particularly are discussing how to deal with the returnees. Some for sure have been radicalized and now used to violence, might want to commit other acts of terrorism. Many already threaten new bomb attacks. All have been exposed to acts of violence, both as perpetrators and witnesses. Many who are known to have returned are under surveillance, and some have argued that they should be stripped of their citizenship. But surely, this will only lead to many going underground and not seek help for the PTSD that many of them doubtlessly suffer and that so often leads to other acts of violence.