Unlike most other main rivals, notably Red Bull and Ferrari, McLaren - whose British driver Button won the 2012 finale in Brazil - did not simply 'evolve' last year's car for the new season.
Red Bull's outspoken Dr Helmut Marko said this week he didn't understand why McLaren changed so much between 2012 and 2013.
Mark Webber agrees: "This sport is hard enough - and the cars difficult enough to understand technically - without taking risks unnecessarily."
Indeed, things are going well in the Red Bull camp.
"I'm very happy with the car, with the balance, how it feels," Germany's SID news agency quotes world champion Sebastian Vettel as saying.
McLaren's contrasting philosophy, however, was that development of the 2012 concept was almost exhausted, and so a new base was necessary.
Button explained that, had the Woking team simply evolved the MP4-27, "I think after three or four races you would realise that you're at the end of the development curve with it".
The problem right now, though, is that McLaren is struggling to "understand where the car is".
Button continued: "At times it feels good, at others it doesn't."
There are whispers in the paddock that arguably the best-performing car at the present moment is the evolved Lotus.
Romain Grosjean did a four-stop race simulation on Thursday, and team boss Eric Boullier beamed afterwards that - towards the end of the faux race - the E21 seemed to be showing potentially grand prix-winning pace.
"It (Lotus' pace) certainly was not bad," Sauber's Nico Hulkenberg told Auto Motor und Sport.
"But Lotus was also good here during winter testing last year."
McLaren's Button thinks it's too early to say if there's a standout performer in 2013.
"I don't know," he said. "All I know is that no one is massively standing out."