Yep, alot of variables. Bent shaft, out of balance shaft, to short of shaft, binding pinion angle issues, tranny and engine bolted together in a stressed condition alignment, etc.
The shaft does pull out on decell, if it comes out to far it can shake and break the tail housing.
Or a combination of any of the listed variables can stack up and break the housing too.
With your spool mount I think you would have enough driveline flex to avoid a stress break. Id focus on blance, length and staightness.