1. ISIS is extremely wealthy, which means it can afford fake passports, first-class air-fares, hotel rooms and automatic weaponry.
2. The refugee line is months long, and provides uncertain results:
A terrorist from Raqqa would have to walk hundreds of miles, find a people-smuggler, cross the Mediterranean without being apprehended or drowned, trek through Southern Europe, dodge the authorities, get over the razor-wire at the Serbian border, travel to Calais, scale the Eurotunnel fence without being arrested, cling to the underneath of a truck without falling off and being killed, and then land in the UK.
Now imagine ISIS wants to inject more than one fighter into a Western country. The refugee line gives no guarantee that expensively-trained fighters, primed and ready to die for the cause, will even get to their target country alive, let alone before their martyrdom-zeal wears off.
Therefore, this idea is militarily absurd and operationally impossible.
3. Daesh strategy is to inspire home-grown fighters; and failing that, to ensure their terrorists are in the target country legally. To risk an operation on the grounds of visa problems or illegal immigration would be ridiculous.
We must remember that the 7/7 bombers and the murderers of Lee Rigby were British, the Paris terrorists were EU citizens, the Charlie Hebdo killers were French, and the 9/11 attackers were in the United States legally.
4. Conclusion: The idea that ISIS sends terrorists along with refugees is a racist fantasy.