Would metal stabilizers, rudders and couplers have failed under the same or similar circumstances? They never have.
What Are the Lessons Learned and What Questions Do They Give Rise To?
At the cost of 500 lives and millions of dollars in lost aircraft, what can be learned from the crash of Air France Flight 447 and the series of emergency incidents and other similar airplane crashes that led up to it?
Is Composite Structural Design and Manufacturing Technology Sufficiently Mature To Be Used in Critical Structures on Passenger Aircraft? In cooperation with NASA’s Aircraft Energy Efficiency (ACEE) Program to improve the fuel economy of commercial aircraft, Boeing commenced an experimental carbon/epoxy flight service program in the early 1970s and included a limited number of experimental elevators on 727s and horizontal stabilizers and spoilers on 737s. 
"The experience gained from the ACEE programs provided the confidence needed by Boeing to select CFRP [carbon fiber reinforced polymer] for the Boeing 757, 767 and 737-300 control surfaces in the late 1970's" 
Although some Boeing 737s have experienced rudder problems, including two fatal crashes; none involved aircraft with plastic stabilizers. Rather, the problem with unexpected rudder movements was traced to a faulty hydraulic servo valve, and the metal tail fins did not separate from the fuselage during flight. 
While Boeing was still experimenting with the use of composite materials in commercial aircraft, Airbus began to extensively install plastic materials in the construction of its first A300 series as early as 1974, introduced a composite tail fin box in its A310 series in 1978, and began delivery of the A320 series with an all composite tail fin in 1988. 
NASA’s efforts to explore the effective use of composites in aircraft design and manufacture in the U.S. was transparent, papers were presented, and information and experience was openly shared. European research and experience in the design and use of composites was more closely held, and it is less clear what kind of foundation work Airbus did in developing its use of composites. 
In 2001, NASA assessed the state-of-the-art in the design and manufacturing of large composite structures in a paper by Charles E. Harris and Mark J. Shuart, which concluded that:
"Composite structural design and manufacturing technology is not yet fully mature for all applications. There are 3 key factors that contribute to the lack of maturity of the design and manufacturing technology. These factors are the lack of a full understanding of damage mechanisms and structural failure modes, the inability to reliably predict the cost of developing composite structures, and the high costs of fabricating composite structure relative to convention aluminum structure. While the technology required to overcome these uncertainties is under development, these factors are barriers to expanding the application of composites to heavy loaded, primary structure." (emphasis added) 
Mr. Shuart states that "all of us (at NASA) are proponents of the effective use of composites in aerospace," and that the Boeing research and testing experience "makes us feel good." He believes "in the right material for the right application," and the main "question is how do you design and meet loads?" 
According to Mr. Shuart, there are places where it may be inappropriate to use composite materials instead of metal such as where there is a "banging around" or "excessive wear," as in joints, hinges, or bearings. 
Mr. Shuart believes it may be useful and prudent to do a "hard scrub," or thorough review, of the design loads used by Airbus in the design of critical structures in its aircraft. He is of the opinion that "failures are more likely a design, rather than a composite problem." 
Regarding Airbus’ use of composites in rudders, couplers and vertical stabilizers, Mr. Shuart said, "What you’re asking is a good question." 
In the Use of Composite Materials, Should Aircraft Designers Anticipate the Unexpected in Recognizing That Composite Materials Used in All Critical Structures Will Experience Extreme Stress At Some Point? As we have seen, a variety of causes have been found in the various emergency in-flight incidents and crashes involving the damage or loss of composite rudders and tail fins on Airbus aircraft.
In the case of American Airlines Flight 587, the primary cause was attributed to pilot error in the "unnecessary and excessive rudder pedal inputs" that caused the rudder to move beyond "design limitations" and cause the plastic tail fin to be broken off the airplane. However, it must be expected that, at some time during the lifetime of an aircraft that a pilot may accidently push a little too hard on the rudder or that the rudder actuator mechanisms may fail.