That's true, but countries like Austria yielded with just a few quality samples from a single project basal clades for major E-V13 subclades. At the same time, the testing at NGS level is surely not much better in countries like Austria, Czech Republic, Slovakia, Hungary, Romania and Moldova. Additionally, in these countries there was not just a contraction of the population, but a large scale replacement. Like in Austria minimum 50-75 percent of the average Roman times lineages being replaced by Germanic and Slavic newcomers. If you calculate from these numbers, you see a drastic rise of today not as common clades of E1b, G2, J, I2 and non-Germanic R1b. This really changes the region, which was rather at the Western transitional zone to the more R1b dominated sphere. And I don't think that the whole variation can be explained by Roman times movements, even less so for Czechia, where the numbers for the pre-Germanic/pre-Slavic lineages, even if assuming some E-V13 were coming with Germanic and Slavic, is not that fundamentally different. Yet it was outside of the Roman sphere and the same pattern just thins out to the North, there is no real cut off.
So I think it doesn't matter that much that the current results are skewed in this respect, because the disadvantage for the Balkan region was surely much less important than in the remaining, more Northern regions of the former E-V13 distribution, where in some regions a near total replacement took place. Like if some Urnfield groups in Poland had higher rates of E-V13, just as a hypothetical example, so many groups rolled over them, that the question is what's supposed to have survived there anyway? The results are skewed for the British testing population however, that's for sure. Because from Britain we have almost everything, if any group would be that well tested, even Germans are at a much lower rate, things would be much more clear. Unfortunately we are a long way from that.
I think the Balkan situation being best described as a retracting of groups which tried to evade the incoming steppe nomads and Slavs in particular. Some of these groups united with Slavic incomers, that formed new Southern Slavic lineages, but others tried to evade the new Slavic rule by pushing themselves Southward or retreated to harder to access terrain, especially as mountain pastoralists.