I may have used the wrong word, but I meant durability in the sense of ability to withstand cycles of changing stress. Introducing a discontinuity is detrimental in many cases. Even if the weld itself is strong, the fact that it is stronger - and stiffer - than the surrounding material can result in fatigue around the weld because the stress increases in the surrounding material when the weld doesn't flex.
You said it yourself, a one-piece part is stronger than a welded part. It isn't because the weld is weaker but because the weld and the surrounding material isn't equally strong.
But I didn't major in material science och mechanical design but in electronics so I could be entirely wrong.
