I want to add one thing to all the answers already posted.
For background, there is a popular Twitch streamer and Youtuber named DougDoug, who is brothers with one of the creators of The Stanley Parable (Originally a HL2 mod, then a standalone game made using Source, recently re-released using Unity), Davey Wreden. On one stream, Doug had Davey on stream and had a Q&A about Stanley Parable and game development ([Here is a cut-down VOD](https://youtu.be/REnFIJhVA-g))
Near the beginning, Davey was talking about Source and porting the game to Unity. He said one of the reasons they ported it to Unity was because Source is good for one thing and one thing only: level design. You can do basically whatever you want level wise but anything else is pretty tightly constrained.
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