The old junker said “not to get involved with robots” because they “bring nothing but trouble”, which in my opinion can be interpreted in two ways
From the beginning, Mr. Customer talks about different kinds of war machines (robots), describing his encounter with one and how he barely escaped it. In this occasion, he remembers what the old junker said to him about robots. The robots who are still alive at this point are mostly military weapons, which either are (possibly) under control by warmongers or independent parties, or just gone into auto mode altogether like the Crab.
We now come to think that the robots are not to be trusted, and this reflects on Mr. Customer’s very first attitude towards Yumemi. It was an attitude which melted quickly due to Yumemi’s personality, but his first reaction was of distrust more than anything else.
I personally believe this is most realistically the interpretation the old junker was trying to convey: he could have met some killer robot and barely escaped just like Mr. Customer, and thus labeled robots as “troublesome”.
Fast forward towards the end of the game. Mr. Customer is crying over Yumemi’s body, and he remembers the old junker’s words: Mr. Customer got involved with a robot, and now he’s out in the open, with anti-personnel machines and possibly warmongers about to rain hell on him.
She brought him almost nothing but trouble.
Except he’s fine with that, because Yumemi had given him back his humanity.
It’s very poetic how the perspective shifts, and how the irony marks the contrast between the start and the end of the story.