Before revision:  However, because of the heavy-tailedness of this distribution, estimation of its tail quantiles is not a trivial task, which becomes even more difficult when there are outliers in the data, or data is pooled among several sources.
After revision:  However, because of the heavy-tailedness of this distribution, estimation of its tail quantiles is not a trivial task, in particular if one takes into account the heavy tails of this distribution, the possibility of singular outliers, and, moreover, the fact that data is usually pooled among several sources.