"Control" can also include things like pinning down one variable while you work on another. Then when you find out what you need to know, pin down the other variable and work on the first.
I like D-76 and XTOL. I'll probably never stop using them but, if Diafine locks down most of the variables associated with development, I can work on film exposure and printing, knowing that my development will always be consistent. Then, when I understand how that works, I can take the information gained and go back to the other developer and experiment with them.
So, in this case, Diafine might give me "control" because I know that I my development will always be consistent.
The downside might be that I have less latitude for varying my exposure but, at least for experimental purposes, that could be a good trade-off for consistency.
Bottom line: I hope Diafine will be one more tool for me to keep in my tool box (cupboard) for use when I need (or want) it. Thus, what seems to take away control gives me more ways to handle my workflow as situations demand.