Very interestng discissuion , Thanks Helimut and Oblivia , I learned something .
Now in the discussions we had in my area (ATC) some years ago , the biggest threat was identified in the "professional" UAVs in the 1,5 to 25 Kg category. Not the small " recreational " ones. There are 20+ Kg UAVs flying out there outside of many regulations.
One example given was that of the Spanish Electricity Provider ( REE) who utilizes UAVs to monitor their power lines. long before the Spanish authorities , and EASA, had published any rules on how to operate them.
A collision with those will put down an helicopter , a GA aircrfat or even a Canadair anti fire bomber operating at the same altitudes as those UAVs.
TV stations ( especially those reporting sports event) are also starting to use large UAVs, and mixing them with ( e.g police or from other TV stations ) helicopters was also identified as a serious issue.