What is "the trap" behind adding upgrades to the current F1 cars?

Max Verstappen has been very critical in recent weeks: One minute his RB20 does pretty much everything he wants, only for that to turn into an ‘uncontrollable’ car barely a few minutes later. For example, qualifying at the Italian Grand Prix could be used a case study, with Q2 being fine and then in Q3, the balance was suddenly totally gone. Verstappen and Red Bull are not alone in this, with teams like Mercedes and Aston Martin also struggling this season to find the right windage for their cars.