Is a waiver of the right to appeal a sentence enforceable?

"Waivers of the right to appeal a sentence are presumptively enforceable." United States v. Arevalo, 628 F.3d 93, 98 (2d Cir. 2010). They are "unenforceable only in very limited situations, such as when the waiver was not made knowingly, voluntarily, and competently, when the sentence was imposed based on constitutionally impermissible factors, such as ethnic, racial or other prohibited biases, when the government breached the plea agreement, or when the sentencing court failed to enunciate any rationale for the defendant's sentence." Id. (quotation marks omitted).