Do they slaughter other mods? No. If a mod is well made it will become popular. Those mods are popular because they did new stuff first. They have nothing against competitor mods, and Mekanism is a good example of it.
In my opinion though, Universal Electricity feels fragmented and incomplete, and doesn't really mesh well with most mods besides themselves, and they don't have enough content to fill every niche that other mods do.
They also dont update very efficiently. Basic Components is still a floppy broken piece of crap, MMMPS still has no 1.6.2, all of UE and NEI didnt get along untill like a week ago. They are very disorganized and VERY bad at keeping on top of glitches or flat out broken builds. They just seem to release without regard as to whether or not they will work.