What is strength of lumberyard compared to Unity or Unreal?

As far languages go, you can use C++, Lua or visual scripting (called Script Canvas).

Amazon Lumberyard has peaked my interests way more than any other game engine, because Lumberyard has a some cool systems which are gems, slices, and terrain tools. The gems are very unique because they can be used around all your games, and you don't have to remake them which is the same as slices. With the terrain tool you can use materials to paint onto the terrain which is not allowed in unity and I am not sure about unreal, but Lumberyard is fully free to use and produce games with (excluding the AWS Software) which is very nice as an added feature towards Lumberyards ever growing popularity with every new version that comes with it.

p.s. Lumberyard has a lot more potential than you might think your just have to work with it. :)

One of the strengths is from a financial standpoint, it is free & you can either set up your own servers or use AWS & Twich which are top products in their respective industry, instagram used AWS when they started & Twich is industry standard to get gamers to buy games. I have been using this engine for over a month with great results, i just looked deep into the lua api available in the editor & the game examples provided with the engine, it is all there. Now i am looking forward to a release this year. I have a Blender3d pipe with this engine & dumped unity.