Hey !
I'm using the new Arcgis js api 4.8, and try to use the new feature of the Heatmap layer.
I try it both on feature layer from url, and feature layer created from graphics array source.
All others renderers work on those feature layers (uniqueValue, classBreaks, simple..) and only the Heatmap renderer does not work.
I try use with with custom values on the renderer and without any custom options and both ways does not work for me.
Any ideas why ? In the examples of the heatmap, there is only use in CSVlayer.
Do you have a sample of one not working? Make sure you have WebGL enabled, as that is required for Heatmap renderer.
Since you need to define the stops manually, you can try creating it in the webmap and looking at the output renderer object like this sample. heatmap