While looking into some approaches for creating relocatable code I came across Z80ASM by SLR Sytems.
This is an assembler + linker for CP/M from 1984 which makes our assemblers, created years later, look like toys.
With it's $199.99 price tag (see Computer Language issue 6), this utility is definitely targeting the professional developer.
For the Spectrum the lack of a standard for disk storage could be used.
For the SAM - while Comet was great at the time - in hindsight, it could have been a lot better. Editor and assembler should have been decoupled and a linker should have been added - along with all the other common 'professional' features of much older assemblers.