I don’t know if I use harp libraries the wrong way, but I often have frustrations with low rumbling sustains. Almost like a sustain pedal on a piano.
Especially if I pluck in the lower range, it’s just overwhelming long sustains.
I have Elysium Harp and the Harp in Pianoteq.
Any tips, what am I doing wrong? 
On the real instruments, I think you’re supposed to damp the strings with the hand as needed to avoid that problem. Basically the same situation as with cymbals, chromatic percussion and the like. Some have mute pedals for that. (Like an inverse sustain pedal, as they obviously can’t have key operated per-note dampers like a piano - though some VIs emulate that behavior, to make life easier for keyboard players.)
It doesn’t seem like Elysium has a literal damping feature, but you can implement it in a limited way by reducing the Release time. The reason this works is that the Release time is ignored when you play with the sustain pedal down, so set up like that, it kind of behaves like a piano.
It still seems to add new voices when plucking the same string repeatedly with the pedal down, which is not optimal, but that’s kind of how it is with samples; that bit is not realistic no matter how you turn it. Unfortunately, it doesn’t support half-pedaling (analog CC64), and once the release envelope is triggered, it runs to silence, so “short pedaling” doesn’t work either. I suppose you could work around it by automating the release control instead of using the sustain pedal mechanism to reduce the build-up in such situations, but that of course leaves the last few notes ringing non-interactively.
I’ve been using Orchestral Tools solo harp from their Berlin Inspire library and I don’t notice anything like that. In fact I want to say it almost doesn’t sustain enough, so maybe I need to use the sustain pedal. I never thought about it since a real concert harp doesn’t have a sus pedal, just the tuning pedals.
Do you have piece where we can listen to it?
I have honestly hardly used harps much earlier, except for glissandos (scale runs) in the high range where this is not an issue. Now I started experimenting with playing more full range arpeggios, and this issue was apparent.
Yes I see what you mean. I find it hard sometimes to use both piano and harp to near each other in the same piece I only tend to use harp in the lower register for itself, a little more upfront, maybe instead of a piano as accompaniment to say a violin.
Do you roll of the low end with filter/eq on the harp?
I’m not good at mixing but I cut every library around 80 in the low
I think it really depends on what you want to achieve and how want the instrument to be used.
If you only use harp as a solo instrument then you shorten the release. If you use the harp as a „fill-in“ instrument, I would cut the lows more and shorten release even more, as no one will hear that in context of the entire piece.
Another thing is the orchestration. Sometimes it’s just better to avoid things like you said you have and instead using instruments that work better. It will be always a part of samples.
One 
thing you need to remember. Libraries are build to work on their own and not with the intention that you will use them with others. Sounds weird, but I don’t think that a developer thinks about other libraries that other developers produce. So knowing this, it’s always your job to make sure they work like you want them too. Meaning: some libraries you can change like the release times for each single note / sample. It’s extra work, but theoretically you could program like higher notes stay as they are while the lower register starts to lose „release“ when playing lower notes.
I know it kind of sucks…