Will it hold up or is it too brittle?

Not sure of its bedding capabilities but knew a guy who broke out a 2" X 3" chunk of metal from the bottom of his aluminum motorcycle engine. After a thorough cleaning and roughing up the surrounding metal he JB Welded the pieces back in place and when I still saw him it held up well 2-3 years. I haven't seen him or talked to him in 10 years so not sure how much longer it held up.

It will hold up- I bedded my Howa M1500 .223 with it.

Even moreso than using Acraglas, you must use a release agent or heavy grease on all metal parts of the action; otherwise, you'll glue the action to the stock.
You should also remove the trigger or at least cover it with tape and apply grease to the tape.

A buddy that shoots benchrest uses JB Weld just for that purpose (gluing the stock to the action). I think he's nuts but apparently a lotta benchers do just that on a regular basis.

Works for me.

There are lighter fillers available to not add as much weight.

Bedding is minimal and the felt weight difference is unnoticeable.

Maybe this time.
Not always.

Devcon 10110 or nothing