AFAIK, MyHeritage has no mtDNA SNPs in their Raw Data. Unless there is another version of MH raw data that FTDNA and MH have not released.
Ancestry has only a few, so FTDNA will have limited success with those uploads.
23andMe has about 4000 mtDNA SNPs, so FTDNA should do okay there, but probably not any better than 23andMe itself.
I have not seen any numbers as to how many mtDNA SNPs are in a complete FTDNA Family Finder Raw Data.
Ancestry has only a few, so FTDNA will have limited success with those uploads.
23andMe has about 4000 mtDNA SNPs, so FTDNA should do okay there, but probably not any better than 23andMe itself.
I have not seen any numbers as to how many mtDNA SNPs are in a complete FTDNA Family Finder Raw Data.