They seem to have gone with a unique aero-concept that only worked in the wind tunnel but didn't deliver on track performance. Many have commented that the MCL33 has too much drag slowing it down but this was vehemently denied by Eric Bouillier; "If it was only about drag we would have fixed it." Due to his history you have to take whatever Eric said with a grain of salt but the tone seemed to imply that he knew the problems with the car had little to do with Aero related drag.Now that the season is over, it is said that there is a fundamental issue with the mcl33 that prvented it from being developed and it was discovered too late for mclaren to have made a "B car"
The car ran a high rake, simplified sidepods and turning vanes, a suspension that maximized airflow through the coke bottle and a unique nose concept that I would speculate was all about sealing the floor and maximizing the effect of the diffuser. Having run Honda engines for years McLaren no doubt spent quite a bit of time trying to extract downforce from aspects of the cars that didn't suffer from lack of HP. The MCL33 was afterall designed for the Honda engine. However in the attempt to improve on the strengths of the MCL32 they appear to have gone in the wrong direction.
The last minute change to the Renault engine had a number of impacts on the design. The coke bottle shape had to be altered to accommodate the bulkier rear-end of the Renault engine (it's turbo is at the back of the engine instead of being in the 'V') which likely led to a number of run-on changes to the cooling, body work, suspension design, sidepods, etc.
What is known is that they have a fundamental problem with the gearbox. The Renault engine decision was made too late in the process and the gearbox couldn't be redesigned in time. The crank shaft of the Renault didn't line up the same as the Honda engine so the gearbox had to be hastily modified and compromised to make it fit causing reliability issues. They also had the wrong set of gear ratios in the gearbox which cost them performance and since the ratios are locked-in prior to season start they couldn't change them.
This is speculation but the gearbox alone might be leaving as much as .2 to .3 secs on the table. I also suspect that they've had suspension issues. The MCL33 has an unusual suspension configuration and if that has caused them any problems then a more conventional geometry may not have been an option as that too would require significant changes to the gearbox casing and modifying the aero package of the entire car.
The innovative nose the put on the car doesn't appear to have delivered performance as expected. As can be seen by removing the extra fairings in the last couple of races. These parts either added too much drag or had a negative effect on stability.
In my opinion the MCL33 was a dud, a car that was an aerodynamic gamble that proved to be fundamentally flawed to begin with that was then exasperated by upgrades that didn't deliver and the last minute switch to the Renault engine. By mid-season McLaren knew full well that the car needed a serious redesign and after the heads rolled in management they decided to save their money and get a head start on the MCL34 instead.