The devs are well and fully aware of the issues with fullbody, by the way. They had a side-project going on to port the fullbody system to VRIK (which is what IK Tweaks does), but the project was put on hold in favor of other things. If the IK was good already, why would they bother starting such a project?
Also with the new recent hires, I have confirmation that several people in the VRChat team literally use IK Tweaks themselves. But the individual developers don't have the kind of freedom to just decide and pick up and fix something like that.