Which LaTeX engine should a mixed-legacy project standardize on: pdfLaTeX, XeLaTeX, or LuaLaTeX?

A project accumulates documents over years: some use inputenc/fontenc with T1 encoding, others need Unicode text and system OpenType fonts via fontspec, and a few rely on packages whose documentation assumes pdfLaTeX. The goal is to pick one engine as the default for new and migrated documents.

The tension is that each engine trades something off. pdfLaTeX maximizes compatibility with older templates and packages but lacks native Unicode and system font support. XeLaTeX and LuaLaTeX handle UTF-8 and OpenType fonts directly, yet identical source can produce different hyphenation, line breaking, and pagination across engines, and some legacy packages silently degrade or fail. LuaLaTeX additionally offers Lua callbacks for programmatic document manipulation, which has no equivalent elsewhere. Assume a current TeX Live release; behavior is version-sensitive.

Given these constraints:

1. Which engine is the most defensible default when both legacy package compatibility and Unicode font support are hard requirements?
2. Does committing to LuaLaTeX for its scripting features create lock-in that complicates sharing documents with collaborators on pdfLaTeX-based journal templates?
3. What minimal compile checks best reveal engine-specific differences before migrating an existing document?