2017 was Ferrari 'B team' Haas' second season in F1, finishing eighth in the constructors' standings.
"We are very solid under certain conditions, but we lack consistency," Magnussen told Auto Hebdo.
"When we are good we are really good, but when we are bad we are terribly bad. The 2018 car will not have this quality."
Magnussen, 25, therefore tipped Haas to keep developing and aim for the kind of results fourth-placed Force India have achieved in recent years.
"We are young but we're growing," said the former McLaren driver.
"It's amazing what the team manages to do despite our lack of experience.
"I think in general the small teams can do a good job - look at Force India. Haas has a bright future ahead of it."
The new season kicks off with the Australian Grand Prix on March 25.