Nice job. I don't think you will have any corrosion issues but I think the forces that caused the original cracks will cause the joint to fail faster this time due lesser strength in the braze filler. I think all the flexing the guard does must be concentrated at the crack line and will continue to do so. A fusion weld would last longer.
When repairing something that cracked, the repair method is secondary to figuring out why it cracked the first time. If you can't find and fix the root cause, you'll just fix it again later and your customer will not be happy.
I am assuming in the picture we are looking at the outside,visible part of the mudguard. Agreed it will probably crack again without reinforcement. It looks like the crack probably started at a kink at the outer rolled edge and worked its way up thru vibration or just bumping it. A small plate welded on the underside starting from the rolled edge should stop it from starting there again. On that type of repair I would drill some aprox 3/16 holes in either the fender or plate where they nest together and clamp and plug weld them together as well as the perimeter weld. My 2 cents.
Yes, the crack developed in a period of yeaeaears. I made sure that the rolled over part had a big rounded lump, also on the inside to prevent the crack to start.
It would be even better if I had reinforced it. But we wil see how long this will hold,
