It's an important concern, but I think mitigatable - especially since we're not neccessarily talking about actual self contained replicating machines, and instead more likely automated factories producing drones - a multi step process where everything has to go right for replication, and into which safeguards can be engineered/studied. Basically, the process needs to be designed to fail due to entropy without external intervention/maintenance.

If we become advanced enough to consider building such things, we can engage in a serious investigation of the risks - for now it's just speculation.