The problem with Flexible Response was that the linkage between battlefield to tactical to strategic weapons inevitably, in ever6 exercise, escalated to general response rather than de-escalated. which is why the INF Treaty was eventually signed to get rid of them, and the linkage risk.
Russia has, if you read the paper above, introduced the equivalent of Flexible Response for homeland defence, with all the risks entailed - and which has also, inevitably, as the need the range of weapons to implement the policy, their reintroduction of tactical weapons and the demise of the INF Treaty.