F-35 is the latest technological wonder-weapon of United States Air Force. It is advertised as a "do-it-all" LO fighter, which is supposed to carry out a list of diverse missions, such as interception of enemy aircraft, fleet defense, tactical bombing of static targets, close air support, ground interdiction, reconnaissance and intelligence gathering, acting as a forward air controller, carrying out SEAD and low-altitude penetration. It is a replacement for Harrier II, F/A-18, AV-8B, Sea Harrier and A-6. It is also pushed as a replacement for F-16 and A-10, despite vastly different roles and requirements.
As with so many "omnipotent" weapons, instead of doing everything equally good, it ended up doing everything equally bad. To explain: every of listed missions has its own set of specific, and often contradictory, requirements.
While Lockheed Martin and USAF are spewing bullshit, F-35's designation as a "Joint Strike Fighter" says it all; it was intended as a bomber and low-altitude penetration and strike aircraft, not as an air superiority fighter. Proof of that can be easily acquired by simply taking a look at F-35 itself: small wing and high wing loading are ideal for low-level penetration, by making aircraft less susceptible to air fluctuations common near the ground. However, fighter aircraft rely on a lift from the wing to turn, using excess thrust to overcome the drag during the turn; thus, good fighter aircraft has to have low wing loading and high thrust-to-weight ratio; exact opposite of what F-35 has.
Close Air Support aircraft has to be heavily armored, slow and armed with cannon in 27-30 mm range, in order to be able to fly close to the ground, identify targets on its own and pull off a precise Close Air Support. While "smart" bombs and missiles are assumed to be perfectly precise, reality is often different - guidance systems are easily jammed, and all complex weapons malfunction often. Thus CAS aircraft has to be able to go low and slow and attack targets with minimum of sensors and communications required. It also has to be able to loiter for a long time, providing continuous support to troops on the ground; with F-35s high fuel consumption, any notion of F-35 doing such thing is a wishful thinking. F-35 also cannot identify targets on its own.
Tactical bomber has to be able to carry relatively large amount of ammunition and strike as many (static) targets as possible. With F-35 being able to carry only four bombs at most, plus two AA missiles, it means that far more F-35s will be required as opposed to F-16s.
Air controller has to have two crew members; F-35 only has one.
Moreover, any aircraft has to be affordable enough to be procured in quantities large enough to carry out missions, at an affordable cost. At procurement costs that might be as high as 352,8 million USD per aircraft (from 2011 cost of 305 million USD per aircraft), and per-aircraft weapons system flyaway cost ranging from 197 to 238 million USD (for comparasion, Eurofighter Typhoon costs 120 million USD w.s. flyaway, and 200 million USD unit procurement), F-35 is anything but affordable. F-35's maintenance cost can be estimated at 48 800 USD per flying hour, going from F-22s maintenance cost of 61 000 USD per flying hour. In short, F-35 costs up to 4 times as much to buy, and over 10 times as much to maintain as F-16.
All of that means that small F-35 force will be completely unable to maintain sufficient presence in the air in face of cheaper, simpler 4th generation aircraft.
F-35 has to rely on unproven dream of BVR combat to shoot down the enemy. BVR missiles did have a Pk of 50 % against "soft" (non-maneuvering, not using ECM) targets in a war where BVR force outnumbered the enemy. However, even in such circumstances, majority of BVR missiles were themselves fired from within visual range. Moreover, emerging technologies, such as air-to-air anti-radiation missile, will make radiating in war even more impractical than it is today; and any aircraft that uses active sensors to find targets is very easy to spot far before it can spot the enemy. Meanwhile, HF and VHF radars - which have already been used to detect aircraft in war (UK's Home Chain in World War II) can as easily detect stealth aircraft, as can increasingly common IRST. Unique radar signal can also be used to solve IFF problem, so important in BVR combat - and enemy using air-to-air anti-radiation missiles can easily force everyone to shut down radars.
In both BVR and WVR, only chance F-35 has is to "launch and run". However, it's maximum speed of Mach 1,6 means that enemy fighters can simply eject BVR missiles, catch up with it and shoot - or gun - it down.
F-35 has also been compromised by different service requirements - for example, Marine's VTOL requirement meant that fighter had to be short and fat, increasing drag. In the end, despite all compromises and accomodating performance penalties, three F-35 variants share only 30% of all parts. And much like F-111, F-35 can only serve as a bombing truck, being too underperforming and too vulnerable in any other role. Both aircraft are extremely flammable and have heavy lack of maneuverability, making them vulnerable to anti-aircraft artillery and SAMs. As a result, both can only fly bombing missions in low-threat areas.
F-35, like F-22, is limited to highly-visible, very vulnerable concrete air fields. Due to extremely hot and strong engine exhaust, STOVL/VTOL variant may never be able to fly from amphibious landing ships without destroying deck in the process.
It seems that realities outlined are being realized: UK may halve its F-35 buy, and other countries, such as Australia, Canada and Norway, are also looking for alternatives, despite all the PR, spin-doctoring, diplomatic threats and bribes Lockheed Martin has used to secure foreign sales. US Navy is also thinking about backing from the program.