Cites 1 failed democracy: Venezuela.

Amanda Taub on Venezuela:

Venezuela, for instance, enjoyed the highest possible scores on Freedom House’s measures of political rights and democracy in the 1980s. But those democratic practices were not deeply rooted. During that apparent period of stability, Venezuela already scored as deconsolidating on the Mounk-Foa test.

Since then, Venezuelan democracy has declined significantly. In 1992, a faction of the Venezuelan military loyal to Hugo Chávez attempted a coup against the democratically elected government. Mr. Chávez was elected president in 1998 on a wave of populist support, and he immediately passed a new constitution that consolidated his power. His government cracked down on dissent, imprisoned political opponents and shredded the country’s economy with a series of ill-planned economic overhauls.

Wikipedia on Venezuela:

The election of Carlos Andrés Pérez in 1973 coincided with the 1973 oil crisis, in which Venezuela's income exploded as oil prices soared; oil industries were nationalized in 1976. This led to massive increases in public spending, but also increases in external debts, which continued into the 1980s when the collapse of oil prices during the 1980s crippled the Venezuelan economy. As the government started to devalue the currency in February 1983 to face its financial obligations, Venezuelans' real standards of living fell dramatically. A number of failed economic policies and increasing corruption in government led to rising poverty and crime, worsening social indicators, and increased political instability.

Economic crises in the 1980s and 1990s led to a political crisis in which hundreds died in the Caracazo riots of 1989, two attempted coups d'état in 1992, and the impeachment of President Carlos Andrés Pérez (re-elected in 1988) for corruption in 1993. Coup leader Hugo Chávez was pardoned in March 1994 by president Rafael Caldera, with a clean slate and his political rights reinstated.

I would argue that any democracy test that notes a banana republic like '80s Venezuela as "enjoying the highest possible scores" is perhaps not a useful test.

I also really want to see the methodology on any study that purports to provide consistent results across fifty years, six countries and three continents.