It’s probably model by model. My brother inkjet was just as bad as a hp. Wouldnt print black if color was low, error codes from generic ink.

I got a brother laser that doesn’t complain about generic toner.

Not on my MFC-L2740DW at least. I've used generic toner from Monoprice just fine.

No. I have two Brother laser printers here (HL-L8360CDW and HL-5470DW) and have been using generic toners since the OEM toner cartridges ran out. I have never seen any warnings.

Maybe, but I own Brother printers for >10 years and I haven't seen such a warning yet.