v0.15.0 is a breaking change, so there will be small differences to the API. The major features added here are multi-entry support and preserveModules support. These may cause some breakage to tsdx.config.js users due to output.dir usage (as opposed to output.file).
While there are some ways to make this backward compatible to users who don't use these features, the differences between the modes may cause confusion and may not be optimal. We may also introduce other tiny changes to naming to help get to a more ideal state or to make things more backward-compatible.

Rollup 2 support has now been added as part of this milestone as it similarly can cause some breakage to tsdx.config.js users.