The drawbacks of long and equal length are that they introduce more surface area, and more volume. The energy to the turbo is a combination of temperature and pressure (don't use the "E" word anybody). Longer cools the exhaust much like a radiator, and also takes longer to pressurize when you go from low flow to boost unless ...
you have a divided turbine housing, and you have connected our 1,2,and 3 to one side, and 4,5, and 6 to the other. Then you can convert the pressure of the pulses to energy in the turbine, which is a beautiful thing. In practice it is not usually achievable in a boxer engine. The three equal length pipes have to come together at one side of the divided turbine housing with no single pipe collector in between; otherwise the pulse conversion efficiency is greatly reduced.
Faced with these problems, most engineers turn the dial the other way and make the manifold as short and small as possible, abandoning equal length. In this way the thermal losses and volume are minimized.
