I have been using the ATK connected to my Apex for about 6 months now. Overall, I am happy with it, and got a second one for a new tank I'm setting up. However, there have definitely been issues.
1. The low level optical sensor failed in CLOSED position after less than 3 months. This caused the sump water level to drop as the ATK thought it was full. Fortunately I caught the situation before the pump sucked air. I replaced the defective sensor with a spare I had. I do not know for sure since I didn't bother, but to get it 'repaired' under warranty, they probably would have wanted me to send in the entire sensor/float assembly; nope. These optical sensors, on the ATK and also on the DDR, have had a horrible track record for reliability; everyone I know personally who uses them has had at least one fail. I'm actually not convinced that they're meant to be used entirely submerged at all. But they are convenient, and easy to place in even tight locations.
2. The pmup is also reported to be somewhat fragile, especially if you use kalkwasser in your ATO reservoir. Premature breakage of blades and corrosion of the magnet have been seen. Also electrical failure with unpredictable effects on Apex operation. In part for these reasons, I have a) moved the kalk to its own small reactor; only pure H2O in the reservoir now, and b) added a separately programmed solenoid in the ATO line, so that even if the pmup has a seizure, its runaway output will be blocked. Also if you need the siphon break, for example with a Red Sea Reefer, kalk in your ATO is already a setup for disaster, even if the pump functions perfectly.
3. The OPEN/CLOSED sensitivity of the optical sensors is pretty good, and does the job just fine as far as providing small frequent RODI additions to keep your salinity very steady. However, in my case, I haven't found that I can depend on Apex + ATK to dose kalk in the very even doses I'd like, when I try to base it on optical sensor detection of evaporation. Set to minimum interval of 30 minutes, sometimes I'll get a dose for 10-15 seconds, and sometimes for 30 seconds or more. I've tried a number of things and can't get it tighter than that. So I switched to timed ATO dosing using the OSC command, which is working extremely well to both handle evaporation and dose kalk evenly. With this approach, the optical sensors will allow or not allow the dosing, but they don't trigger it directly.