The problem is that once the illegals become unionized, they are unemployable. They are only employable now because they will work for far lower than even non-union wages.
The moment the illegals start a union, their employers will go back to employing legal non-union workers.
The money the union people command will go way down once the illegals join unions.
The illegals may make a little more money with the unions.
But it will be lower wages or no jobs at all for union workers.
Union members will still have to pay dues.
Union members will still be counted as a voting block.
Why else would the unions be interested in the illegals?