The deal with epoxy is:

The slower it is about setting up, the tougher it will tend to be when it's done.

Adding filler material or not has no effect upon this rule. Slow-setting epoxy with steel filler will be tougher than fast-setting epoxy would be with the same filler.

I've seen five-minute epoxy that you could dent with your thumbnail, 24 hours after it was set. The only thing worse that that, I would suppose, would be the hot-melt bedding system that some manufacturers resort to.