You can't really compare Unity with Construct3 because it is used for completly different stuff! We work with both, Unity and Construct. When it comes to 3D projects we currently use Unity because it has way more options. BUT if we do any 2D projects, especially for mobile, we always used Construct3 because it saves months of work! It is way easier and faster to do 2D in Construct3 than in Unity and everyone who has ever worked with Unity knows that!
To the pricing / free thing:
Unity only wants money if you want to remove the "Unity-Logo" on start-screen of your project or if you earn more than 100k / year with your project as I remember and it's not revenue based, it's just 1k / year or something like that if you do more than 100k / year.
I guess Unity is making their money with the big asset-store and also with their ads-network (Unity Ads) and not with their game engine.
And yes, you can use Unity for free, but if you compare the work for a specific 2D project (like 2 months with C3 and 4 months with Unity), the time you save is worth way more than the fee for the C3 paid plan.