Just had a 70k damage game earlier in it but for the most part the weaknesses are major and difficult to overlook.
1.) as mentioned AA is non-existent even for tier 4 and as a result you are a big, juicy target for carriers and this ship sees 2 carriers per side -very often. Players who know the ship or seal clubbers will pick on you so be warned.
2.) the range is a real issue because all BB at your tier and up can outrange you as can most cruisers. This is bad because you are a battlecruiser not a battleship. If you had the typical IJN gun range (listening WG?) and range was more in the 14-15km range like Myogi for example then you would definitely want to stay at 14-15km. The guns have good HE as mentioned but AP penetration seems terrible vs your counterparts and a strong ship like Emperor Nicholas will crush you in almost every category. Myogi, Wyomings, New Yorks and especially Kongos will be a challenge. They can see you first and shoot you first. Turning at your gun range of 12km in front of battleships will be painful if they haven't all fired and at that range most people don't miss.
My advice would be to wait for hopefully a deal on Fujin if you can't afford an Atago.
The Ishizuchi can be effective but is rarely fun imo although with a little range buff that would change. Once I got other premiums I stopped playing it, I make more in other ships as well.