A year ago playing a precon with them gave me a pretty insane winrate, probably 70%+. These days I think precons are relatively reasonable. But if you tasked me to build a deck with a precon budget but as powerful as possible, I'm sure I could make something pretty effective.
That said, I don't really like budget-restricted builds because it's tedious to track prices while building. I'd much rather build set-commander or PDH or whatever so I don't need to reference any outside sources to ensure my deck remains within the limits.