All sensors in current canon DSLRs are deficient. Both, APS-C and "full frame 36X24".
They are significantly sub-par compared to sensors found in competitors cameras. They lack 25% to 50% in resolution and at the same time they lack up to 2 EV of dynamic range, dependung on ISO setting.
Canon's sensors are as deficient today as a car engine would be that delivers only half of the acceleration at half the fuel efficiency of competitors engines.
Canon has only gotten away with this up to now, because
* real life differences are mitigated by best jpg engine and
* all other parameters of their DSLRs, especially user interface - are fully competitive and
* most importantly, because many customers are "locked in" by their high investments in not-deficient canon lenses and system components (speedlites).
Canon is in a dangerous spot today: better sensors can be had now in smaller packages and can be combined with canon lenses. If sony gets its act together, and manages to quickly improve their defivient Mirrorless ff cameras (af, battery charge, EVF) and canon continues to ignore mirrorless or only delivers half-assed products, they could quickly become obsolete. Nokia and blackberry should serve as warning examples.
Nonsense — from the sensors being "deficient" to customers being "locked in". The sensors perform amazingly well in a wide range of applications. Canon sensors long ago met the quality threshold of very demanding professionals as well as millions of amateurs. Of course the sensors can be improved — everything can be improved — every detail of every camera system can be improved. But calling the sensors "deficient" is nonsense. Canon sensors are described as "deficient" primarily by a few anonymous critics on internet forums. They typically point to DxO tests in support of their claims, not to any real photography.
As for being "locked in" — switching brands is easier than ever. One no longer has to go to the camera shop and get 50% or less of the resale value. Nor does one have to advertise in the newspaper classifieds and hope a willing buyer might notice. With so many online resources for selling used gear, a system switch can be accomplished very quickly. It's easy to recover nearly the full cash value of one's gear and apply it toward comparable gear from another brand. If dynamic range and resolution are so important to someone, and makes a substantial difference to their photography, then being "locked in" is just a sorry, empty excuse for continuing to bash Canon on forums.