Overambitious
/ˌəʊvəræmˈbɪʃəs/
Definition 1 of 1
Pro Tip 1/3Realistic Limits
Use 'overambitious' when someone ignores realistic limits in their plans or actions. 
The startup failed due to overambitious expansion without the necessary funds.
Pro Tip 2/3Unrealistic Expectations
It implies that expectations or targets are too high to be achieved in a given circumstance. 
The overambitious targets set by the manager demoralized the team.
Pro Tip 3/3Negative Outcome
'Overambitious' often suggests that there will be negative consequences due to excessive ambition. 
Her overambitious schedule led to stress and health issues.