[Mister Moose]

I have a fair bit of experience with epoxy and wood in a marine environment. Keep in mind that if your oak or ash has gone soft what you have is not "soft wood" what you have is rot. It no longer has structural strength. I agree that epoxy tends to be brittle, and likely won't hold nails well.

If there are gaps or voids, consider an additive that adds strength. The WEST system has a whole series of additives that either increase finishing sandability, add strength, add viscosity, etc. You can use it in a consistency from maple syrup to mayonnaise to putty. It depends what you add. DO NOT mix in large quantities, and be careful above 80 degrees ambient, it cures with heat, and it creates its own heat. You can get a runaway in a big potful, and that's exciting, not in a good way.

If you use an epoxy, consider small self tapping screws with a pre-drilled hole, it will hold far better. You could also use the epoxy as a filler/glue, and scarf in more real wood in the more damaged areas. That gets you real wood where the rot is without replacing the entire piece. Not likely in this application, but be aware that epoxy can degrade in UV, this is very true for WEST epoxy. It needs to be shielded from sunlight or at least painted.

Hard to say without photos, but there are options.

*Edit: Keep in mind when you repair rot, it almost always goes further than you thought.