Problem 1: Extra vertical lines in the bottom of table with overlayarea. Tried solutions in Too long vertical lines in table, but changing |c|c| to cc is not okay for the looks of the table.
Problem 2: The table heading wobbles with overlayarea in slide 1 and 2.
MWE:
\documentclass{beamer}
\begin{document}
\begin{frame}
\frametitle{Title}
\framesubtitle{Subtitle}
\begin{overlayarea}{\textwidth}{2cm}
\begin{table}
\begin{tabular}{|c|c|c|}
\hline
\only<1-3>{
\textbf{Sl. No.} & \textbf{Heading1} & \textbf{Heading2}\\
\hline
}
\only<2-3>{
1 & Content1.1 & Content1.2\\
\hline
}
\only<3>{
2 & Content2.1 & Content2.2\\
\hline
}
\end{tabular}
\end{table}
\end{overlayarea}
\end{frame}
\end{document}
MWE (same as previous, but without any newline, tabs or space):
\documentclass{beamer}
\begin{document}
\begin{frame}
\frametitle{Title}
\framesubtitle{Subtitle}
\begin{overlayarea}{\textwidth}{2cm}
\begin{table}\begin{tabular}{|c|c|c|}\hline\only<1-3>{\textbf{Sl. No.}&\textbf{Heading1}&\textbf{Heading2}\\\hline}\only<2-3>{1&Content1.1&Content1.2\\\hline}\only<3>{2&Content2.1&Content2.2\\\hline}\end{tabular}\end{table}
\end{overlayarea}
\end{frame}
\end{document}

overlayareadoes not eliminate wobble effect. Though I solved it using|p{width of longest text in this column}|. I expectedoverlayareato take care of it. – ItzMe Nov 16 '17 at 20:29\onlynodes in a "safe" place so not just after an\hlinewhich is why I restructured to always add then inside the cell at the end of a row, you will note I added\to your headings here to force them to be wider than the content (or as you say, you can use fixed width columns) – David Carlisle Nov 16 '17 at 20:36\now. Good option, did not know this. Thanks. – ItzMe Nov 16 '17 at 20:51