One earthly concern is that, because of this unified interface myth, management will think that once you integrated with one broker, the others come free because FIX.
Oh, you need another week or two with the other broker? Why? We spent so much just to get the industry standard API.
Sorry I'm too jaded to appreciate sequence numbering and the industry standard logout message format that fix provides at the given time.
About your point on rest, I do acknowledge the difficulties you're mentioning, of every broker requiring different rest messages to do the same thing. I've come across brokers which accept single orders and grouping in different messages. The delay and errors it causes is insufferable.
The problem is, there has not been a body enforcing unity across brokers. Fix is a nice try, but it is left alone in the woods without any restrictions as to what brokers can or cannot alter. I would like this pointed out and heard of more and that's it.
