The hardest part is the work. As in, the manual labor.
"If you TIG weld, you will get to sit. I have to stand all day." This was my reply when someone asked which method I recommend learning. She thought I was teasing, but it was an honest reply. Seriously: If you TIG weld, you will get to sit!
What they might not tell you about a brazing torch, is that it is heavy. My first impression of brazing was dominated by this. It is heavy, and yet I had to learn to hold it up and wield it as if it was the lightest thing in the world, for what seemed like an eternity at a time. I was so happy when I finally got used to its heft. But then a week went by when I did not use the torch, and today it was heavy again. It felt like my upper arm, and not the lug, was being lapped at by the flame. (If you TIG weld, you will get to use a tiny little torch, light as a feather. In my dream the welding torch resembled a colibri bird...)
Building this frame is endless work. My hands are a mess. At the end of the day I am drained, unable to do or think about anything else. I just want to go to sleep, and do it again tomorrow. Why? Because hard work feels good if you submit to it. And because building a bicycle frame is fun. And because I'm almost done. For now!