Tax reforms may help the United States score more investments from both local and foreign companies, but those benefits could be offset by the Trump administration's protectionist trade policies, a major accounting firm said Friday.

Since President Donald Trump took office, the U.S. has pulled out of the Trans-Pacific Partnership and is re-negotiating the North American Free Trade Agreement and the Korea-United States Free Trade Agreement.

"(What) everyone has to keep an eye on is the trade issue. A lot of it is also dependent on NAFTA and KORUS and all these trade deals keeping the supply chain to be able to invest that way, and that's a big issue for 2018," Mark Weinberger, global chairman and CEO of EY, told CNBC.

Pulling out of those trade deals or setting more stringent requirements for the entry of foreign goods and services could reduce the incentive for companies to invest in the U.S. — which is the opposite of the intended impact of tax reform.

The restructuring of the U.S. tax code — the largest in 30 years — reduces the corporate tax rate to 21 percent from 35 percent. Companies including Fiat Chrysler and Foxconn have announced billion-dollar investments in the U.S. to take advantage of lower taxes.