Are ATA and SCSI commands implemented differently from vendor to vendor?
The SATA protocol stack is a part of the T13.org's ATA stack. This stack is defined in several documents for architecture, commands and physical transfer (SATA and PATA).

For example the ATA8 command set defines mandatory and optional commands. You can can test the availability of features in the IDENTIFY DEVICE response. The ATA8 standard defines only command and data structures as an interfaces. So the internal implementation is vendor specific.

Regarding NCQ:
NCQ defines up to 32 slots for outstanding requests, but many consumer HDDs / SSDs support less (e.g. 4). This saves hardware resources - if implemented with separate buffers - or reduces queue management in software.

Server HDDs and SSDs are not limited.