As you can see, it depends on your location and your insurance plan, as well as some of the coding used by your provider. In Wisconsin, it doesn't matter what kind of doc you are, ie there is no bias against plastic surgeons. That's because botox for migraines can be performed by rehab med docs, neurologists, chronic pain docs, anesthesia, or internal medicine. My suggestion, however, is to stick with "core" physicians for those with the most experience with botox: plastic surgery, facial plastic surgery, oculoplastic surgery, or dermatology.
Overall trends regarding insurance coverage are on the decline. In my experience here in Wisconsin, we fought for coverage and saw steady iimprovement from the late 1990's through mid 2000's. In the last three years that has clearly been reversed, to the point that it is exceptional to get coverage at this point. Sad, but true. Especially since it is effective for those properly screened, in the majority of cases. Most patients also had to first prove "failure or significant side effects" using other meds like beta blockers and imitrex.
Call the benefits number on the back of your insurance card and ask them directly if Botox for migraines is a covered expense, as well as if there are restrictions (provider, frequency, proof of prior treatments).