As a long time Nebula owner, I'm not very happy with this plug-in. It was a nightmare to get all up and running, find the needed libraries, get the keyfile and such stuff. Ok, since some time they changed the website, so it's easier to find all downloads. But this is not the best solution for a commercial plug-in. Acustica should provide something like a better installer and maintaining tool to install the free libraries later. A idea of such a good tool is the Imageline library tool.
The GUI is average and for most things not very usable. This is the problem with such a GUI which should be usable for many different sampled devices. EQ, compressor, tapes... All have the same faders only with customized labels for the parameters. Better would be a GUI description "language" or a XML file where the libraries could load there optimized/own interface. The library and the short names are a bad idea. I had to ask the developer of the R2R libraries to get an idea about the meaning of the short names. This is a very big limitation not to have longer names and a description.
I bought Nebula in the past because of the Doc Fear EQ (D.W Fearn). But because of the limitations of Nebula3 it's not usable like a "Pultec clone". I wished it has a interface with all knobs like the D.W Fearn on one page and all settings there. So I must search in the library with the cryptic names and load a part (only the shelf EQ or a combo provided by the author).
CPU usage and latency is very high. I tested also to change the internal buffer size but this will not help. So it's more a plug-in I use for 1 or 2 instances.
The sound is ok but I prefer a good emulation instead of the sampled devices. The limitations of Nebula are for me a show stopper.