It wouldn't be a stretch to say they checked the BCS records from the -21 and found out the system's too sensitive, as Guld just thinking about crashing Isamu made it crash Isamu... Yes, BDI made him crazy, however, the YF-21 can't function without it. Think about it! You can't collect data fast enough through just your eyes to run the BCS on the thing, therefore BDI is necessary to keep you up with what's happening.
Moving on, Hikaru was stationed aboard Megaroad-01, which, I believe, is missing, non? Max was stationed aboard the SDF-1, defending New Macross City while the SDF-1 was refit, and later, placed in charge of the NMC Macross 7.
Mac+ - II (That's a bit of time, ain't it? Like 60 years' time...), I've no clue, besides Mac7 and Frontier...
Back to the YF-21, the -19's known to be cheaper, requires no extra training or augmentation, and has no control system bugs.