The downsides? The ergonomics of the A9 are not as good as the latest Sony models (i.e., it is a little smaller, the grip is smaller, buttons are smaller), but it's not awful. The dynamic range is not as good as, say, my A1 or A76, but it is still decent. Images are 24MP, which can be good and bad for some (if you want resolution) -- the good is that file sizes on your card are smaller than an A1/A7R series camera. The AF is not as updated as newer cameras (e.g., the eye-AF is not as sticky) but it is still very fast and accurate. The back screen pulls out and tilts down, and is not a flippy screen like you see on newer cameras. If I recall, the viewfinder refresh rate is not as high as newer cameras like my A1. The USB port on the side of the camera is Micro-USB, which is not the modern USB-C standard, but also is more friendly to using a wired intervalometer. You can charge the camera by plugging the USB port directly to power (e.g., to a USB power brick or a wall charger) whereas the batteries in the newer cameras absolutely need to be plugged in to a Sony charger. These are some of the differences I noticed when the A9 is compared to my A1 and A76.
For the money, I think it's a killer camera, even after all these years.