Use 'disadvantageous' to describe choices likely to result in negative outcomes or setbacks. Competing without preparation can be disadvantageous for your career.
Pro Tip 2/3
Implicit Comparison
Implies a comparison to something better, even if the better alternative isn't mentioned. Their refusal to update the software was disadvantageous compared to competitors.
Pro Tip 3/3
Careful Usage
Avoid using 'disadvantageous' to describe people; it can seem judgmental or offensive. She avoided calling his lack of experience 'disadvantageous.'