izze Hi ! my thought process for using backwardEuler for deer was that its fat ratio or volume is less, So didn't wanted to be get distracted by high frequency. Reason was that to get better collision i need to increase the subset and BDF2/TR-BDF2 integrator will give higher frequency jiggle . Though one can lower mass value to reduce the effect.
May be if it was animal like Hippo , walrus or Elephant which are huge and have good fat ratio, Than i would have used BDF2/TR-BDF2 as it would give more details to Fat/skin sim. As mentioned by stanp the simulation will be more lively.
@Jake You can try to play around with these parameters as for my deer fat sim tweaking these parameters helped me a lot. Like "collisionPointSpacing" somewhere between 0.05 - 0.025. and did increased "contactStiffness" of zTissue and zBones. to get the good collision. And also played with "inertialDamping" of zTissue.