Things to remember about 2009 is that it was the start of a new "narrow car / KERS / slicks" era so everybody had to integrate a new type of engine into a new type of chassis. Whereas in 2018 there is a lot of rules stability so if you are one of the teams changing a PU, that is going to be a disadvantage versus those who aren't.
Indeed, quite right. Also noting that Brawn didn't even integrate KERS into their package, they ditched it completely, for simplicity. Thus leaving the majority of other front-running teams to struggle half the year with it's implementation. So in that sense they had an easier time with packaging than almost everyone else that year.
Back then, they had cooling in the sidepods for the ICE. Now they have cooling all over the place for the ICE, the MGU-H, MGU-K, Control Electronics, Battery pack... And all the V8s were more or less the same shape and size. Packaging was similar after years of convergence. They sat about the same place in the car. There was none of this is-the-turbo-at-the-front-in-the-middle-or-hanging-out-the-back-of-the-vee business which moves the whole block forwards or backwards.
In contrast, McLaren had a very complex job to do in a very small amount of time. So, albeit with the benefit of hindsight, turning up and testing and Melbourne with an under-cooked car should come as no surprise. But the car will soon be done cooking, hence the bullishness from all involved.