Caution is good but willow is actually very safe. You can use it green or dried. It does take a bit more to reach effectiveness, especially if you are using whole plant material instead of the tedious inner bark scraping. My primary concern is herbicide and pesticide spraying. While it is true that chemical concentrations vary, this is true with ALL home remedies. There is only so much you can do to control that without a lab. But who wants to do that. For each batch you will need to start at a low dose and work up to what works for YOU.
Feverfew is a good Migraine medicine. It will need to be used as a tonic rather than as an acute symptom medication though.
For tension headaches there are other herbs that will work better than willow: lobelia, valerian, chamomile, cramp bark and many others that work on the nervous system.
Also for your information there are many willow type plants that can be substituted: aspen, poplar, birch are most of them. Others have been mentioned above.