Given all the technology used to study the eruption, why is it a challenge for researchers to estimate the total volume of lava erupted by Kilauea in 2018?

Because it has an Effusive eruption.

Explanation:

The Kilauea volcano in Hawaii erupted a large amount of lava starting from 1983 to 2018 the longest eruption ever. Since it had an Effusive eruption it was slightly difficult to measure total amount of lava erupted.

Normally geologist measure the total amount of lava erupted by checking examining those erupted from ancient eruptions.

The reason for the difficulty in measuring the amount of lava erupted by effusively erupted volcanoes is that they tend to have long pauses and the slow outpouring of lava is not continuous in time. So it is not easy to determine when one ends and a new one begins.