First there was 5th ed. I never played it but I own the book. Then sixth edition came and we got Ravening Hordes (army rules for everyone that were planned to be superceded by army books). That's when I started playing. I did quite well but I was 13-14 and we were all were newbie as possible. Then the LM got a temporary "army book" in a White Dwarf magazine. That was a serious nerfing over Ravening Hordes. Then we got a 6th edition book that made us strong again just about when my friends drifted away from the hobby and I followed suit.
I missed seventh edition, but I checked out Temple Pyramid Vault (the old LM forum) for nostaligia. Seemed to be mid tier. Cavalry and large model units with Fear were the kings of seventh. We had one of those two things.
Jumped on the boat again with 8th edition via the 7th edition book. We were in the bottom of the top tier running almost entirely on Slann power (extra power dice all the way!). Then the 8th edition book pretty much kept us in the same place though it removed the tendency to have mono-builds somewhat, though we lost ground by standing still because with the exception of the first three or four army books (which got the shaft), most armies got a power upgrade with the new book while we held steady.
Then came End Times. I think if you use the magic rules in Khaine we are a top tier army on par with the combined Legion armies though we are pretty much forced to take High Magic Slann. If you use everything but the Khaine magic then we were knocked down to the mid tier. Maybe a lower tier. We didn't get any special characters and we didn't get the gestalt armies like Chaos, Elves, and Undead did.
We seem to be a top tier in Age of Sigmar. Assuming you try for some semblance of balance. The armies with access to summoning have an edge over armies that do not. You can field a small starting army with a Slann (or multiple Slann), dictate Sudden Death terms and them make up for your numerical short term by summoning stuff left and right.