Problem when using the specifier `b` of `xparse` with tabulars

As Phelype commented we looked at this and might add the special table group required to hide nested & but meanwhile you can add them using the new environment hook feature.

documentclass{article}
usepackage{array}
NewDocumentEnvironment{MyEnv}{b}
  {$begin{array}{cc}#1end{array}$}
  {}

BeforeBeginEnvironment{MyEnv}{{ifnum0=`}fi}
AfterEndEnvironment{MyEnv}{ifnum0=`{fi}}

begin{document}

begin{MyEnv}x&yend{MyEnv} 

begin{tabular}{cc}
smth & begin{MyEnv}x&yend{MyEnv}
end{tabular}
end{document}