Alternative to detokenize working with special unicode characters

It's a bit more complicated than using \detokenize, particularly with IL2. By the way, do you really need it?

Here's a way that preprocesses the input so to change _<> into proper commands.

\documentclass[12pt, oneside]{book}
\usepackage[a4paper,top=2.5cm,bottom=2.5cm,left=3.5cm,right=2cm]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[IL2]{fontenc}
\usepackage[slovak]{babel}
\usepackage{nameref}
\usepackage{xparse}
\usepackage[hidelinks,breaklinks]{hyperref}

\ExplSyntaxOn

\NewDocumentCommand\mynode{mmmm}
 {
  \mynode_main:nnnn { #1 } { #2 } { #3 } { #4 }
 }

\seq_new:N \l_mynode_input_seq
\seq_new:N \l_mynode_output_seq
\tl_new:N \l_mynode_node_tl
\tl_new:N \l_mynode_text_tl

\cs_new_protected:Nn \mynode_main:nnnn
 {
  \seq_set_split:Nnn \l_mynode_input_seq { ; } { #3 }
  \seq_set_split:Nnn \l_mynode_output_seq { ; } { #4 }
  \tl_set:Nn \l_mynode_node_tl { #1 }
  \mynode_replace:N \l_mynode_node_tl
  \tl_set:Nn \l_mynode_text_tl { #2 }
  \mynode_replace:N \l_mynode_text_tl

  \mynode_paragraph:VV \l_mynode_node_tl \l_mynode_text_tl
  \label{ \tl_to_str:n { #1 } }

  \begin{itemize}
    \item vstupné~hrany:~
    \seq_map_variable:NNn \l_mynode_input_seq \l_mynode_node_tl
     {
      \mynode_print:N \l_mynode_node_tl
     }
    \item výstupné~hrany:~
    \seq_map_variable:NNn \l_mynode_output_seq \l_mynode_node_tl
     {
      \mynode_print:N \l_mynode_node_tl
     }
  \end{itemize}
 }

\cs_new_protected:Nn \mynode_paragraph:nn
 {
  \paragraph[#1]{#2~(\texttt{#1}):}
 }
\cs_generate_variant:Nn \mynode_paragraph:nn { VV }

\cs_new_protected:Nn \mynode_print:N
 {
  \texttt
   {
    \tl_if_empty:NTF #1
      { - }
      { \nameref { \tl_to_str:N #1 } }
   };~
 }

\cs_new_protected:Nn \mynode_replace:N
 {
  \regex_replace_all:nnN { \_ } { \c{_} } #1
  \regex_replace_all:nnN { \< } { \c{textless} } #1
  \regex_replace_all:nnN { \> } { \c{textgreater} } #1
 }

\NewDocumentCommand{\change}{m}
 {
  \tl_set:Nn \l_mynode_text_tl { #1 }
  \mynode_replace:N \l_mynode_text_tl
  \tl_use:N \l_mynode_text_tl
  %\tl_analysis_show:N #1 %%% for debugging
 }
\ExplSyntaxOff

\begin{document}

\mynode{root}{Koreň}{root}{gui}

\mynode{gui}{Grafické rozhranie}{root}{nice_gui;fancy_gui}

\mynode{nice_gui}{Pekné grafické rozhranie}{gui}{}

\mynode{fancy_gui}{This text contains less and greater characters:<>}{gui}{}

\mynode{<test>}{Some text with \texttt{under_score}}{<test>}{}
\end{document}