I am looking for a phrase that is in the same spirit such as "single point of failure", or "bottleneck". Example:
Being two engineers short is <-term goes here-> in this situation. If we find two engineers, the project will meet the deadline.
Another one:
Electric cars are improved enough to be on par with internal combustion engine cars. However, the long charging time is <-term goes here-> behind the problem that electric cars are not as widespread as internal combustion engine cars.
crux, Oxford English Dictionary
The chief problem; the central or decisive point of interest.
Example: 1934 R. Benedict Patterns of Culture vii. 232 The crux of the matter is that the behaviour under consideration must pass through the needle's eye of social acceptance.
The OP's two examples would then be slightly rewritten to read:
Being two engineers short is the crux of our problem. If we find two engineers, the project will meet the deadline.
Electric cars are improved enough to be on par with internal combustion engine cars. However, the long charging time is the crux of the problem causing the slow inroads electric cars are making on internal combustion engine cars.
In rock climbing, the crux is, according to Wikipedia, Glossary of Climbing Terms
The most difficult portion of a climb
There are several terms from project management that might work, but they aren't single words.
Critical success factor: an element that is necessary for an organization or project to achieve its mission.
Long pole in the tent: the issue preventing forward progress on a project. This is more of a slang term, link goes to previous SE question about alternatives
You may be looking for Achilles' heel.
M-W:
Achilles' heel noun
: a fault or weakness that causes or could cause someone or something to fail
I'm trying to lose weight, but ice cream is my Achilles' heel.
the food supply proved to be the nation's Achilles' heel in its defense against terrorist attacks
How about roadblock?
From Merriam-Webster:
roadblock: something that blocks progress or prevents accomplishment of an objective
The OP's examples:
Being two engineers short is a roadblock in this situation. If we find two engineers, the project will meet the deadline. Another one:
Electric cars are improved enough to be on par with internal combustion engine cars. However, the long charging time is a roadblock behind the problem that electric cars are not as widespread as internal combustion engine cars.
A roadblock is a barrier to progress. Once a roadblock is removed, the road is clear -- progress can proceed unimpeded.
critical factor
or
key
Somewhat helpful definitions I found:
critical: of decisive importance with respect to the outcome; crucial: a critical moment. (dictionary.com)
key (adj.): chief; major; important; essential; fundamental; pivotal: a key person in the company; key industries. (dictionary.com)
Examples:
Having enough staffing will be the critical factor in meeting the deadline.
Having enough staffing is key to meeting the deadline.
The long charging time is the key reason electric cars are not as widespread as internal combustion engine cars. (or the critical reason)
