.. is that while they support Thunderbolt 3/4/5 and USB4 – they don’t bloody support USB 3.2 gen 2, 2×2 which is what the majority of external SSD drives on the market support. The result of this lack of support (which is an optional component of USB4) is that external drives are going to be far more limited than the speeds that the manufacturers advertise.
The PC market (e.g. most modern Windows PCs) don’t have this problem. It is incredibly frustrating that Apple have chosen to go down this route to the extent that you are forced to pay their over inflated prices when it comes to internal storage. There are a number of drives (Thunderbolt 3) which overcome some of the limitations, and Thunderbolt 5 drives are slowly making their way – but these are still few and far between – and EXPENSIVE.