Drawbacks of using fragile frames in beamer

there are no important drawbacks. It is also possible to define a new environment with the fragile option:

\documentclass{beamer}

\newenvironment{Foo}[1]
  {\begin{frame}[environment=Foo]{#1}}
  {\end{frame}}

\newenvironment{FooBar}[1]
  {\begin{frame}[fragile,environment=FooBar]{#1}}
  {\end{frame}}
\begin{document}

\begin{Foo}{title} 
\end{Foo}

\begin{FooBar}{title}
\end{FooBar}

\end{document} 

With fragile every contents is written into an external file and read back. Not a big deal but makes compiling slow. However, fragile is needed for verbatim material.

See this thread. Apparently the fragile option is incompatible with the allowframebreaks option, without some awkward finagling. So that's one drawback, in addition to the much slower compilation mentioned in the comments of the accepted answer.