I have experience with the classic Canon 100mm macro as well as the Sony 90 macro. At the time I was using the borrowed Canon I was shooting an A7Riii.
The Canon was running through a Sigma MC11 adapter. While not the fault of the lens specifically, I would find that occasionally the combination would claim focus lock when in fact it was just slightly off. If you’re scanning a lot of film and then sleeving it you can imagine how exasperating it is to discover one of your frames is not quite sharp. You won’t really know if it’s the original image that’s out of focus or your camera scan. And the miss is so slight you’ll only ever see it at pixel level on a computer monitor. Never on the camera lcd. I digitize a lot of film and I can promise you a little bit of that goes a long way.
As a classic lens designed in the 90s it will definitely show its age when put in front of an A7Riii or IV. It seems a lot of the highly regarded Canon glass of the film era does fine at 24mp, which makes sense given the limits of 35mm film. So there’s a question for you: do you think you’re going to upgrade your camera body at some point? If you are it may be better to go for the glass that will grow with you. OTOH 24 mp may be all you need. That’s a question only you can answer.
My goal is to scan my film one time, do it as well as I can do it, and hopefully never touch it again. That led me to pick up the Sony 90 after three months of agonizing over whether I really needed to spend $1100 on it. I read every review on the web and looked at all of the alternatives on the market at the end of 2018.
So what does it offer over the classic Canon? Edge sharpness. Flare resistance. Resolving power, easily keeping up with my A7Riv. And absolutely reliable auto focusing. As a native lens, what little distortion it has is corrected automatically. Rectangles are rendered as perfect rectangles. That’s a big deal for camera scanning.
The only real drawback is the price tag although the lens does offer the ability to be used as a standard portrait lens so if you have a need for that focal length it might take some of the sting out of the price. After using it for nearly 2 years now I can say that I would buy it again in a heartbeat knowing what I know about it now.