Jinx, it depends on the LRM from what I've seen. The Javelis can do so handily without issues, upgrades or support and can better handle the suicide Guardian runs to push them away. Assailants can often hit the Guardians because of how inconsistent the Repulsion field is but have trouble with the aggressive Guardians going towards them. And Illuminators cannot reach the Guardians.
So that's probably the main problem I think, that the long range units still don't have the range, and the other long range alternative (strikecraft) are jokes.
Anyway, I don't think calling people miserable scrubs will sway opinions, honey than vinegar, you know that kinda thing.
To the point, Advent vs Advent fleets I do agree the ability is off because the fighting can be kind of ridiculous (assuming there is actual fighting, and not just Repulsion). Because of Advent's range limitations, two fleets doing this really can't do much to each other. It resembles a Carrier/Flak fleet fighting another Carrier/Flak fleet (for people who have seen such disasters), they can last for 10 minutes with virtually no losses on either side and no progress. When the Advents fight though there will be some minor losses, some ships tend to get broken off from the main group, stray one way or the other, and the ships that get too out ahead might actually get in range of the other fleet's main guns. Even when Illuninators get their range upgrade (T7) they usually have a hard time getting close enough to inflict any substantive damage. So it can be stalemate, have to go around and attack their worlds or try to build a stupid number of Drone Hosts in an attempt to do damage (Guardians have substantial hitpoints, some will be using Shield Projection, likely a Mothership with Shield Restore, so even if there's not flak just making headway would not be easy).
Personally, I think things should wait on a change to see how things shape up. The Guardian has had this ability, unchanged, since release and it's been very underused (along with Advent as a whole), so I think some time to see how things develop may be a good idea.
If Guardian use/abuse becomes prevalent and no suitable counters are devised, I think reducing it's range to maybe 5,000 (it's around 7,000 now) and slightly increasing the Illuinator and Assailant range might help for the Advent vs Advent battles as well as make things easier on Vasari. Any change that improves the Illuminator I am weary of, it's a great unit now if you can get them and improvements to their range could make them too good.
Repulsion probably shouldn't sway capitals either, but that seems like a less important point, except when the Guardians are micro'ed to isolate Capital ships and tear them to pieces (which can be frighteningly effective). Although if Capitals were not affected, I don't see what would stop a player from simply hugging the Capital with Guardians to deprive it of support and then destroying it, since only other Capitals could come to it's aid.
Personally, so long as it can outrange the Heavy Cruisers and can deny an area to them, the ability would still have it's place in an Advent fleet. That's the big, important deal to me, late game you can fight LRMs that focus fire on Capitals and various abilities and support can prolong their lives considerably. But those Heavy Cruiser blobs that can destroy any individual ship they please, including Capitals you invest so much resources, pop cap, and time to nurture into a decent fighting machine is just lost. Repulsion that can deny those beasts from feasting on your prizes is one I think is useful.
This type of thing concerns me though, mostly because Advent is good now after spending so much time out of it, and I like Advent. There are calls to nerf Malice, Cleansing Brilliance, the Malice and Brilliance duo of death and other real staples of the Advent fleets. Basically, the things that make Advent both strong and unique. I think the Guardians should always be able to use Repulsion against Heavy Cruisers and that all the faction's LRM's should be able to combat it. Hopefully if strikecraft become useful, they will also have a place against the ability.