I'm not sure what you mean by "fast scroll," but according to their site, it uses Logi Options+ driver, which offers the same features as the MX. I suspect you mean the accelerated scrolling feature (is that called "Fast Scroll"?), which is supported with the scroll wheel on the top of the mouse. The Ergo is essentially a low-end MX Mouse with the sensor on the top of the mouse with the trackball for movement instead of on the bottom which senses your desk for movement.
What I don't think you'll like is the significantly lower resolution of the laser – it only has 380dpi average compared to the MX's 1000dpi average. This makes the MX much more accurate. It's not that the Ergo is bad in this regard, it's just not as good.
The Ergo also uses a stupid Micro USB charging cable instead of the standard USB-A or USB-C cable of the MX – which is enough for me to pass on it alone (I hate having to keep extra cables around and prefer to just have one USB-C cable plugged in to charge everything, including iPhone, etc.).
I also don't think the relatively small trackball makes it easy to be accurate, or even comfortable to use.
In the end, if I were to switch back to using a Trackball again, I would go back to the
Kensington Expert Mouse - which I happily used for many years.