Are there any negatives concerning use of fermcap as far as attenuation is concerned?
I have never had a problem, but I use it primarily in the boil kettle, not the fermenter.
I’ve used it in the kettle and fermenter as well as both kettle and fermenter and have had no problems with attenuation.
Not that I’ve ever encountered.
I haven’t had any difference in either attenuation or head retention.