Coordination difficulties?
First problem is finding people willing to suffer and die in the process. I don’t know anyone willing to, and asking around would set off a lot of red flags that would be investigated.
Once you find a dozen or so as needed for the Paris scenario, you have to get them to the site without arousing suspicion. Remember, you’re talking a dozen people who aren’t good combination of bright & stable, and moving that many together may, um, arouse suspicion.
Next you have to assure they’re capable of doing the task as planned with the equipment given. This isn’t _hard_, but given the “bright & stable” issue it will take considerable training (heck, those of us who _are_ bright & stable would practice comparable tactics for weeks).
Then there’s getting the equipment. Whatever you think of gun laws, machineguns & grenades are in fact hard to come by in most Western countries.
After getting it, then there’s transporting it. Not particularly hard, but still cases of heavy stuff in high-density urban areas.
Finally, there’s infiltrating without gathering undue attention. Most people would be visibly nervous or otherwise expressive of anxiety & ill-intent.
Well, really finally is actually doing it. Most people wouldn’t, especially people who are bright & stable enough to actually do it if not for the natural psychological brick wall preventing most bright & stable people from doing such things to others.
What happened in Paris wasn’t just “you go here and start shooting at X:0Ypm”. They had hard-to-get weapons, and apparently used well-practiced non-trivial non-obvious tactics. Considering the various possible aborted attempts we occasionally hear about (like today’s canceled soccer game in Germany), seems it ain’t easy.
I hope that if they must try another city it is Dallas. I expect at least half of the people in a gathering are packing, it will be fun to see the results.