the quads I had before were softer than my current vise, but I think it was due to the age of the turbos and where the bags were stored. The turbo inserts were exposed to garage heat vs. the newer bag of inserts are stored in a climate controlled (AC) storage facility.
btw, I change out my inserts frequently on balls that get a lot of lane time. I don't use much glue (placed at 3 and 9 o'clock), but does the glue weaken the ball material over time?
I use and change inserts as often as anyone. I would say that if you use a discreet amount of glue, it will not weaken the resin coverstock. I put my glue more at 8:30 and 2:30 so it doesn't spread upwards; I need the top of the insert free to put shims in and out often.
I usually put the one drop of glue about 1/4 - 3/8" below the top of the ball. One drop of good glue at each side will keep an insert glued in place forever, until you pry it out.
I haven't had one ball crack at the fingers (other than one plastic ball that hit the gutter edge too hard) in 15 years or so. Heck, I haven't had one ball crack, period.