Enumeration and line break tabularx

Two examples for use itemize and enumerate in your table and one example with different table design:

documentclass[a4paper, 11pt]{article}
usepackage[margin=3cm]{geometry}
usepackage{fancyvrb}

% tables
usepackage{ragged2e}   % <--- added
usepackage{booktabs, tabularx}
newcolumntype{C}{>{Centering}X}
newcolumntype{L}{>{RaggedRighthspace{0pt}}X} % <--- new, for the last example
newcolumntype{P}[1]{>{RaggedRighthspace{0pt}}m{#1}}
newlengthcolwidth

usepackage{enumitem}   % <--- new, for lists (in table)
usepackage{etoolbox}   % <--- new, for customization of lists used in tables
AtBeginEnvironment{table}{%
setlist[itemize]{nosep,
                 leftmargin=*,
                 label=textbullet, % <---
                 before=begin{minipage}[t]{linewidth}, 
                 after=end{minipage}}                   

setlist[enumerate]{nosep,
                 leftmargin=*,
                 label=arabic*.,   % <---
                 before=begin{minipage}[t]{linewidth},  
                 after=end{minipage}}                   
                        }

begin{document}
With verb+itemize+
    begin{table}[ht]
settowidthcolwidth{30% acrylamide stock solution30%} % set width of 1st col.
%
begin{tabularx}{textwidth}{@{} P{colwidth} *{2}{C} @{}}
    toprule
    & Transient transfection & Stable transfection  
    midrule
Advantages
    &   begin{itemize}
    item   no genomic integration (no frameshift, or even worse damage to the genome) 
    item   high flexibility 
        end{itemize}   &   
addlinespace
cultured cells
    &   begin{itemize}
    item   electroporation 
    item   lipofection 
    item    microinjection
        end{itemize}   &   
bottomrule
end{tabularx}
    end{table}
    
or with verb+enumerate+:

    begin{table}[ht]
settowidthcolwidth{30% acrylamide stock solution30%} % set width of 1st col.
%
begin{tabularx}{textwidth}{@{} P{colwidth} *{2}{C} @{}}
    toprule
    & Transient transfection & Stable transfection  
    midrule
Advantages
    &   begin{enumerate}
    item   no genomic integration (no frameshift, or even worse damage to the genome)
    item   high flexibility
        end{enumerate}   &   
addlinespace
cultured cells
    &   begin{enumerate}
    item   electroporation
    item   lipofection
    item   microinjection
        end{enumerate}   &   
bottomrule
end{tabularx}
    end{table}
    
or with verb+l+ and verb+L+ types of table columns and verb+enumerate+:

    begin{table}[ht]
begin{tabularx}{textwidth}{@{} l *{2}{L} @{}}
    toprule
    & Transient transfection & Stable transfection  
    midrule
Advantages
    &   begin{enumerate}
    item   no genomic integration (no frameshift, or even worse damage to the genome)
    item   high flexibility
        end{enumerate}   &   
addlinespace
cultured cells
    &   begin{enumerate}
    item   electroporation
    item   lipofection
    item   microinjection
        end{enumerate}   &   
bottomrule
end{tabularx}
    end{table}
end{document}

Addendum: In a case that you like to have only line breaks. IN such a cases you not need enumitem and etoolbox` packages:

documentclass[a4paper, 11pt]{article}
usepackage[margin=3cm]{geometry}
usepackage{fancyvrb}

% tables
usepackage{ragged2e}   % <--- added
usepackage{booktabs, tabularx}
newcolumntype{L}{>{RaggedRighthspace{0pt}}X} % <--- new, for the last example

begin{document}
...

This you can achieve on three ways:

• insert empty lines where you like to break cell's content:

Advantages
    &   no genomic integration (no frameshift, or even worse damage to the genome)

        high flexibility
        &   

• use command `verb+linebreak+

Advantages
    &   no genomic integration (no frameshift, or even worse damage to the genome)linebreak
        high flexibility
        &   

• or use of command par

Advantages
    &   no genomic integration (no frameshift, or even worse damage to the genome)par
        high flexibility
        &   

In all above cases the result is the same: