Garry has publicly expressed regret over choosing Source 2 for this reason, but they might be in too deep now. Generally, the tooling is really great, so I guess that's the primary benefit.
Do you have a source for this? S&box used to use UE4 in the past but as soon as Source 2 became an option immediately jumped ship. There has been a ton of progress since then and I don't see why he would regret switching to Source 2 when no other engine would have made as good of a base for S&box.