This may be drive related (though I have a GGW-H20N and have no problems like this with it personally) and I also don’t know exactly what the situation is with Verbatim and Imation outside Japan, but I do know here in Japan that both brands have moved/are moving their production over to Taiwan. Every MKM (Verbatim) disc sold here I’ve seen lately is MIT and now TDK (Imation) is beginning to sell bd-rs labeled as MIT too. So basically you are getting CMC or Ritek discs I believe if they are made in Taiwan (someone correct if I’m wrong) which may be the reason for the quality discrepancies. Especially with bd-res which are more fickle anyway.
And in case someone is curious, the only brands I know that are still 100% MIJ are panasonic, jvc, and sony (and possibly maxell) for blu-ray blanks.