In a damning new report, they say that any results achieved so far "remain fragile."
In their analysis, the auditors assessed whether European Commission and European External Action Service assistance was proving effective in supporting the reforms in Ukraine.
They examined the areas of public finance management and the fight against corruption, as well as the gas sector, between 2007 and 2015.
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EU-Ukraine cooperation advanced in the wake of the 2014 Maidan events, say the European Court of Auditors (ECA), but the challenges faced by Ukraine "still heavily affect the reform process and the risks posed by the former and new oligarchs remain high."
The report was published at a news conference in Brussels on Tuesday.
Ukraine is one of the European Union's largest and politically most important neighbours. However, institutional and political instability, governance issues, and economic and financial difficulties still predominate in the country.
Ukraine has, say the auditors, a record of poor economic performance since independence in 1991.
The EU responded to the 2014 crisis with a package of €11.2bn over seven years.
From 2007 to 2015, EU financial assistance had comprised €1.6bn in grants, half of this in the form of budget support, and €3.4bn in macro-financial loans.
But the report concludes that the EU had "allocated and disbursed large amounts of money rapidly and without first agreeing its strategy."
In the gas sector, it adds, "The design of EU assistance to improve governance and security of (energy) supply included conditions that were vague and difficult to assess.
"Monitoring sometimes lacked high-quality data and EU-Ukraine joint reporting did not provide a sufficiently detailed explanation of the difficulties experienced. Before 2014, EU assistance had little impact on the functioning of the Ukrainian gas sector and produced mixed results in terms of security of supply."
Speaking at the report's launch, Szabolcs Fazakas, the member of the ECA who was responsible for the report, said, "EU support for Ukraine remains a work in progress, despite good efforts by the Commission."
Fazakas added, "At the time of our audit, there was a strong political commitment to public administration reform. But management changes jeopardised the reforms and low salaries created openings for corruption. Further steps are needed to meet objectives."
The auditors reported "some tangible and sustainable" results in public finance reform and an improved anti-corruption framework. The 2011 visa liberalisation action plan is also cited as an example of improvement.
However, their report goes on, "The results of anti-corruption measures remain to be seen."
The auditors make a number of recommendations for improving EU assistance to the country, where a bloody civil war is still rumbling on in eastern Ukraine, including putting "greater emphasis on the effective implementation and sustainability of reforms."
The EU, they say, should also take steps to make EU assistance to Ukraine more effective in the gas sector.