Ultimately it depends how much faff time you have and how well you want things to work – warranty also rears it’s ugly head, if that is relevant to you.
The best shifting is always achieved by using a system as it was designed to be used – as a system. OK, we are biased as Campagnolo’s main UK SC, but if I were speaking for Shimano or SRAM, my experience and training would lead me to say the same – mixed systems are to some extent always a compromise.
That isn’t so say they won’t work – but in the context of your question you have to take into account that all three main manufacturers use subtly different actuation ratios and approaches to the technical challenges of indexing. The more gears you pack into the space available, the more accurate you have to be and so the more the systems become sensitive to being compromised by an unexpected (by the system) factor being added in.
Shiftmates seem to offer the best of the available compromise solutions to using (specifically) combined Campagnolo / Shimano systems but if you are dealing with new parts, just remember that using any third party part in the system may affect your warranty …
Remember too that the main manufacturers test and test and test on a variety of scenarios that fall within their overall frame design and intended use parameters – individuals may be able to persuade combined systems to work in their particular circumstances but that doesn’t mean that such a solution will work in another set of circumstances.
The more we expect from gearing systems in terms of shift performance, range and overall “slickness”, as in the case of practically every other technical product, the less we can expect to be able to be able to interchange parts and retain those characteristics.
I don’t say that is necessarily a wholly desirable thing but it is “the way it is” …