For an actual race car a header has some advantages.
For a street-driven vehicle, the increase in underhood temp is very annoying - the total surface area of a header is much larger than any casting or fabricated weld-ell. It reduces gas temp reaching the turbine inlet somewhat, so the higher gas speed doesn't work as well as it looks. it makes a cold air inlet (outside the bay) more critical. It's a bit noisier. Obviously, it's more leak and crack-prone just due to the number of joints and even a small leak kills boost.
Bell has some useful comments on dump and bypass location on an L engine and IIRC (haven't read it in 10 years) the end isn't the best.
