It has also been used to great success. The Tampa Bay Bucs, where it gets its namesake, rode it all the way to a Super Bowl win. While it has its place in the NFL, it is not a solid defensive scheme.
The Tampa Two works off a basic "bend, don't break" philosophy. It employs a two-deep coverage over a 4-3 front. The basic idea behind the scheme is to not allow big plays. It is believed that teams will move the ball some but have a hard time sustaining drives without big plays.
Most Tampa Two defenses will blitz significantly less than other NFL defensive schemes. Instead opting to let the players on the field make the plays.
This is the inherent flaw in the defense. It is a defense that's success lies solely in the players on the field. The talent on the defense will take the Tampa Two as far as they can go. But without enough talent, the defense is useless. Just ask Rod Marinelli.
In the NFL today it is critical to get pressure on the quarterback. In most schemes, this is done by twists, stunts, shifts, talented players, and mainly blitzing. Even your best sack artists today only average one sack per game. The other sacks typically come from blitz packages.
The Tampa Two rests on the ability of it's defensive line to get sacks and it's linebackers to plug holes and stop underneath passes. The responsibilities are well outlined and easily coached. Without talent in the position though, a player can be exposed.
A linebacker that is not strong against the run can be run over. He is in a spot and covers an area. He is exploitable. Although good coaches make minor adjustments to combat being exploited, there is only so much that can be done. An injury or two can take a dominant Tampa Two defense and bring it to a grinding halt.
I won't argue the success of the Tampa Two. There is no doubt teams have used it to great success. Ultimately though, it is only because the players on the field were carrying it on their back.