Serotta has a magic ring to most folks; however, like any other builders, there have been faillures.
Know personally of one of the few Ser. Ti tandems built that rear triangle separated. Yes, it was fixed under frame warranty for free.
Same for a Legend Ti single. Broke, was fixed for free.
Have personally had a tandem frame failure (twice . . . once at 50,000 and once at 56,000 miles, but another stressed tube) and one tandem fork.
Sometimes it is just wear, other times . . . ?
Ruy/zonatandem