Drawbacks of using fragile frames in beamer

Question

This is a follow up on beamer's fragile frame as default. What are the drawbacks of setting the default frame to be fragile (or passing the optional fragile argument to all frames). The beamer user guide says

If a frame contains fragile text, different internal mechanisms are used to typeset the frame to ensure that inside the frame the character codes can be reset. The price of switching to another internal mechanism is that either you cannot use overlays or an external file needs to be written and read back (which is not always desirable).

Presumably writing and reading a file takes time and so this increases the time it takes to compile, but are there things that do not work in a fragile frame?

As for an MWE:

\documentclass{beamer}
\def\foo{Something that makes the fragile frame fail is: }
\begin{document}
\begin{frame}
    \foo
\end{frame}
\begin{frame}[fragile]
    \foo
\end{frame}
\end{document}

I am trying to understand what \foo would need to be defined as to cause problems.

Answer 1

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.

Answer 2

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.

Answer 3

The fragile option has some impact on centering:

\documentclass{beamer}

\begin{document}

\begin{frame}
    \centering
    center
\end{frame}

\begin{frame}[fragile]
    \centering
    not center
\end{frame}

\begin{frame}[fragile]
    \begin{center}
        center again
    \end{center}
\end{frame}

\end{document}