Does it apply to any aspect? Is that it?
If I use a multicast attribute it will apply randomly to the first 10 matches only?
I'm not sure if I got this restriction right. Do you mind elaborating it a little? Examples would be well welcome'd.
Starting from PostSharp 4.3, this limitation applies to any aspect. The build will fail if you apply aspects to more than 10 targets (with Multicast or not). So no randomness happens.
If PostSharp 4.3 detects that your project would have been built successfully with PostSharp 4.2 or prior, then the backward-compatibility mode is enabled and the project will be built by PostSharp 4.3 without error. Please refer to PostSharp 4.2 API documentation to check if an API used to be available in PostSharp Express.