Has anyone used Bondo metal reinforced filler ??

No. You are tasked with using it on a magnum caliber and reporting back with the results.
First one to ask, gets the tasker. Army Staff study assignment method.

Why would anyone use it when we have better materials for the job?

Bondo is putty like, easy to shape for use in pattern stock minor changes..

Brownells has a number of easy to use 50/50 mix gel, great for a beginner, read directions..

I like Marine Tex but not for beginners or weak of heart..

I prefer JB weld to any of the others.

Not body putty, but fiberglass works fine. All acraglass is is overpriced, hyped up fiberglass resin.

bondo is both flexible and brittle under light load, so no.

marine tex gray is the answer,

Yes I have used it but prefer Devcon Aluminum or Steel epoxy, they are my go to bedding compounds with Johnsons floor wax as a release agent, but for skim bedding I like Brownells Acraglass Gel or regular Acraglass.
BB

Devcon or similar, Titanium.

Is the one I use now, exclusively.

I bed some very large bores.

For these I use brass bedding lugs I make fir each rifle.

Never had any problems.

Yes, for skim bedding that appears to look like finish is nice for full length bedding, I like clear brownells on a wood stock..bed the stock by hand, then paint with clear Brownells glass bed. Lightly heat the glass with a hair dryer while in the wood before sitting your barreled action in the wood, and no bubbles will show up. Looks great..

I do this to my personal hunting rifles then add cross bolts on the 9.3x62 and larger..or by request..

Phillybond 6 or 6a or Duralloy expoxies. Have used all 3 with good results.

+1 for Devcon. Brian

Devcon