Will researchers from the USA get funding in Health-related projects under Horizon 2020?

As you posted your question in Fit for Health website you might be particularly interested in the rules in the Health topic in which the US have a special status. Due to a bilateral agreement US partners are having automated access to funding from H2020. It is important to understand that this only applies to the topic 'Health'.
You can find this in the footnote 29 on page 73 of the Health Work Programme: 'In recognition of the opening of the US National Institutes of Health’s programmes to European researchers, any legal entity established in the United States of America is eligible to receive Union funding to support its participation in projects supported under all topics in calls under the Societal Challenge ‘Health, demographic change and well-being’.

Only exception is PHC12 (SME Instrument) in which appicants have to be established in a MS or AC (whether subcontracting to a US partner is possible is not specified so I would contact the programme officers about this.)

For all other topics in H2020 , If a grant applicant can convincingly demonstrate that without the participation of a US partner the objectives or impact of the proposal cannot be achieved, the EC can allow the US partner to access funding. But the case has to be really convincing. If you have a US partner who has a specific expertise, a patent, market access or anything that cannot be found within the EU then you may have such a case. What you should definitely avoid is to make the impression that you look for EU funding for research activities and that a significant part of the exploitation or market uptake (hence a large part of the expected turnover) will be generated in the US.

However, if the US partner for example allows a European SME to access the US market, this would be a pro for adding a US partner.

In any case, if you are planning to involve a US partner we strongly recommend to contact the assigned project officer or at least your NCP to get a better idea about the official take on things