Training is not a problem. Anyone who knows the basic of OOP can learn TypeScript in a day.
On the other hand, giving them constraints (because of the type system) ensures their work is not breaking stuff along the way as they learn the ropes of the code base.
From my perspective, setting up a build flow (dependencies, etc) is the biggest problem with TypeScript projects. If you assume that's already done in a given project, then the negative impact is gone and TypeScript only makes it easier.
On the other hand, giving them constraints (because of the type system) ensures their work is not breaking stuff along the way as they learn the ropes of the code base.
From my perspective, setting up a build flow (dependencies, etc) is the biggest problem with TypeScript projects. If you assume that's already done in a given project, then the negative impact is gone and TypeScript only makes it easier.