Pro tip: don't call your people "high maintenance"
Are you a manager? That is, a people manager? Are you someone who has "one on ones" with direct or indirect reports? Do you have influence over their future in a place? Then you probably should stop doing certain things.
It's really not a good idea to suggest that someone is high maintenance. It's even worse if you basically come out and just tell them that. If it's not backed by data, then it's entirely subjective and might not even be true. It only serves to poison any possible working relationship that might exist from that point forward.
There are other variants on this.
Don't imply that one of your people will be "a project". If a person is a project, then you are also saying they are a problem.
In that same vein, implying that you will have to invest a lot of time in someone is also right out. The person who is being told this is going to feel like shit when they hear it.
Don't try to be proud about this. Don't say that you were warned by other people that it would be a big project or that it would take a lot of time. While you might be doing the noble thing, you have just confirmed that a seriously broken situation exists. The recipient of this knowledge can only conclude that fixing it is hopeless because the perception has become the reality for multiple people through repeated assertion. Facts become irrelevant.
Who wants to go to work knowing that they won't get to do their actual job, but instead will have to fight a straw man argument all day long, day after day? I assure you that someone given such feedback will find ways to avoid engaging more, will start detaching, and yes, will eventually leave.