@jseymour84 has a point. Unless a brokerage is brand new or has a small client base, you're likely to find some negative reviews simply due to the percentages. That said, regulation by a reputable financial authority can go a long way to addressing a lot of the concerns you may have.
For example, the CFTC and NFA set the requirements a broker must meet in order to offer forex trading to US residents. Though not an exhaustive list, this membership application will give you an idea of some of those requirements: https://www.nfa.futures.org/registration-membership/who-has-to-register/rfed-applicants-compliance-requirements.html
Particularly noteworthy are the requirements for financial transparency and trade execution accountability. Below is an excerpt from the CFTC site:
The final rules include financial requirements designed to ensure the financial integrity of firms engaging in retail forex transactions and robust customer protections. For example, FCMs [futures commission merchants] and RFEDs [retail foreign exchange dealers] are required to maintain net capital of $20 million plus 5 percent of the amount, if any, by which liabilities to retail forex customers exceed $10 million. Leverage in retail forex customer accounts will be subject to a security deposit requirement to be set by the National Futures Association within limits provided by the Commission. All retail forex counterparties and intermediaries are required to distribute forex-specific risk disclosure statements to customers and comply with comprehensive recordkeeping and reporting requirements.
Due to these strict requirements only four brokers are approved as forex dealer members in the US as of the latest data from the CFTC.