These small changes that could go a big way: In the new refactoring version, the delta sliders are put all the way up to 1000, making the clip black immediately. If instead the sliders are at 200, like in the kdenlive 18 version, this wouldn't confuse users immediately once the effect is added. Currently, all alpha manipulation effects use the "write on clear" operation. This operation means only one alpha manipulation can be used on a clip, since a second one would overwrite the first. A simple change of the default operation from "Write on clear" to "minimum" would fix this problem, meaning a kdenlive user can use multiple alpha manipulation effects of one clip. This would go a long way.
Thank you for this suggestion. See here, I think this is what you want: https://phabricator.kde.org/D20184. I change the bug to whislist.
Change integrated in 19.04, thanks!