A big portion of the miss in the initial hit or miss was due to the introduction of per-tracker IK and max range to ignore distant trackers when IK2.0 came out. It’s now possible, for example, to just not even have your hip tracked at all. So some people who were used to using incorrect “User Real Height” (which affects the range that trackers will bind at) would log in with the hip binding point in the wrong position, bind into FBT with a completely un-tracked hip and assume that is just how bad the new IK was. After many cycles of troubleshooting with users I believe this was the main cause of the two-sided “new IK is horrible” - “I don’t know, works great for me” that sometimes comes up. Using the new calibration visuals should make it clearer when something like that is going wrong.
Another big portion were people who had their base animation layer doing things that disturbed the IK, for example when crouching. I believe mods went as far as to mess around with your animator as well, so people might not have understood what was causing things to shift when they crouched, just that they liked mods better. This should now be solved for those people if they use the new option to disable locomotion animations.
As far as I know, the mod’s spine behavior didn’t allow full locking of both/either head or hip, but was in between. You can approximate this by using IK2’s lock-both mode but looking down when you bind, this will give a bit of a higher binding point on the hip trying to pull it down but to a limited extend (less than lock-head mode would). I don’t consider that our final solution though and I plan another polish pass on the spine behavior, but for now doing that workaround should get you really close to what you were used to.
As far as config options, those have been held behind the new main menu release, but once that’s out I want to get more IK config into the menu, however more spine config (if added) would likely come after the above mentioned spine polish pass. At first, instead it would likely be lower hanging fruit like getting some launch options into the UI and possibly some basic angle config on the knees or wrists that people have been asking for.
The post that Tupper is mentioning is here if you’d like to look at my advice for how to get things to “just work” with the current setup: Developer Update - 5 August 2022 - #35 by Kung
And as Tupper mentioned, if you have any specific feedback / feature requests, feel free to hit up the #ik2 discord channel or especially the IK2 canny