Using TypeScript in tool has really helped our development process - from documentation, refactoring, and code-completion, to the type checking and errors that compliments our tests. We've been converting our tools projects to TypeScript for months now, so this is nothing new.
There are extremely few downsides that we've experienced in practice. The TypeScript compiler is so fast that the project is build be the time we switch tabs to run tests or a command. The main outstanding issue is stack traces referencing the compiled output, but this hasn't been much of a hinderance - the output is very similar to the input, and we're going to look into source map support. I'm not aware what other drawbacks might be big issues.
I personally don't think that the "Use the Platform" philosophy is in conflict with TypeScript, or many other compile-to-JS languages for that matter - in node or the web. For tools, we don't run some huge abstraction over node's processing model, or it's core libraries. On the web, all of the critical platform features are available as with JS: The entire DOM, including Custom Elements, Mutation Observer, Custom Events, Shadow DOM, the CSSOM, and all JS features like classes, Promises, etc. TypeScript doesn't define it's own object model, and always uses JS features directly.
The tools team really needs to maximize it's output, and TypeScript helps us do that while still targeting Node, which our customers appreciate. so there's no reservations from us here. In fact, we grow happier with the decision every day.
Cheers,
Justin