Both imm silver and ENIG provide flat surfaces that are better for fine pitch placement and reflow than HASL.
We know of nothing that makes BGA locations different than other locations when using ENIG solderability protection. More broadly, the primary concern about ENIG reliability has to do with hyper-corrosion of the nickel surface, commonly referred to as "black pad", that makes the pads unsolderable. [Search the fine SMTnet Archives for background on black pad.]
If your supplier can convince you that these boards are solderable and that they have had no "black pad" problems in the past, then consider accepting the boards.