AFAIK the mountains are about as high as Wales? It looks like there's a 600MW storage scheme in the planning stages. That's about a third of the capacity of Wales's largest system (which is the biggest in Europe) and about twice the capacity of the second system.
It's a meaningful contribution I would have thought.
Can't speak for Hungary specifically, but the problem with flat terrain wrt hydro storage isn't just elevation difference - which can be offset with flowrate - it's the large surface area to storage and to depth ratios.
Large lake means more losses, mainly due to evaporation, but seepage is also to be expected. Now put this fact in the thread's context: water shortage/stress...
"Most of the country has an elevation of less than 200 m. Although Hungary has several moderately high ranges of mountains, those reaching heights of 300 m or more cover less than 2% of the country."
"Hydroelectricity is not prevalent in Hungary due to unsuitable geographical conditions and civil resentment. The country's capacity of hydroelectricity was 57 MWs in 2015."
But yeah batteries would be nice.