Hi,
At first, SetMaxScale must be called on MCT thread inside QueuedTask.Run.
After setting new values open layer properties and check is values are specified (general tab).
Max value must be less then min value:
I have tried vb sample from community samples with values:
multiPointLayer.SetMaxScale(1000000)
multiPointLayer.SetMinScale(5000000)
And it works as expected. It could be that your values are to small or not in expected range