The landing scheme is pretty robust, but there are still plenty of opportunities for things to go wrong. There could be one of those famous Martian windstorms that might exceed the airbags' limitations. Any one of the critical operations in the landing sequence could fail, such as detaching from the aeroshell, deploying the parachute, deploying the airbags, surviving the impact, deploying the rover, etc. One thing is certain, Murphy's Law works very well on Mars.

As for terrabytes of data, that one is a certainty - it won't happen. Without checking the comm subsystem specs, odds are the uplink data rate will be somewhat less that 10 kbits/sec. That only works when you have DSN time and the rovers are visible to Earth. A terrabyte is a thousand gigabytes, which is a buttload of bits. Even if it were somehow possible to get signals 24 hours a day, 10 kbits/sec would total 4,500,000 bytes per day. If the mission lasts for the planned 92 days, the total would be 414,000,000 bytes, not even half a gigabyte. Under the more likely conditions, the total will probably be closer to 100 megabytes.