I agree with what you said regarding FTB not being obligated to give release dates, and unless someone has been living under a rock for the last 5-6 years, everyone knows full well how impatient and arrogant people can be in demanding updates, timeframes and features.
I also understand (and agree with) the policy to not release a modlist because by the very nature of developing and configuring a custom modpack will mean that the modlist is going to be in flux. If they were to definitively say "yeah, Thaumcraft 6 is in there" and heaven forbid it wasn't in the final release, it doesn't matter what the reasoning for excluding it was, someone would rage.
As a retired developer, I am keenly aware of the effects of "public input" during the development process. It has benefits, to be sure, but it has a lot of downsides as well. It is very easy to talk yourself into lofty promises you ultimately can't keep, and you will be taken to task for it, no matter how well-intentioned you may have been.
However, it is also important to not neglect your user base either, especially so for open source or volunteer efforts. FTB earned a lot of positive reputation over the years for what it has produced, and that has carried them far. All in all, I think they are doing a very good job with the resources they have. But regular updates are important, even if it literally is nothing more than "We really are still working on this, guys. We've narrowed our reported issues down to half a dozen pages or so, and hope to have more to report next [insert appropriate timeframe here]."
Remember, pack makers and mod authors are frequently not the same thing. Pack makers generally have little to no input on when a given mod author is going to update their mod, or fix a bug, or add a really desired feature to complete a pack. Just as the pack developers are unpaid volunteers, so are the mod authors. Way too many end users seem to let this slip their minds when clamoring to "release it now, we won't mind the bugs, we promise!" "Our users love world corrupting errors! we love losing hours of progress because we didn't want to wait the extra couple of weeks for this to get hunted down and fixed!"
Edit: All that being said, I'd still like to know if OpenComputers is in the pack or not.