The failures are due to hot spots in the top of the block that lead to head gasket failure and coolant entering (often) #3 cylinder. On Youtube Ford Tech Makuloco discusses the EcoBoost coolant issue extensively in one video. You can Google it and watch. The early EB’s had an issue with cylinder cooling passages in the block between #2 and 3 cylinder. Basically the outer bores (#1 and 4) are surrounded on 3 sides with coolant jacket passage while 2 and 3 only have coolant jacket on the front and rear of the bores. This leads to hot spots in the top the block from engine operation but it’s inherent in all in line engines. The EB is aluminum block and head though, so heat transfer and resiliance is an issue (where it wasn’t on older cast iron block and head straight 4 and 6 designs). On the EcoBoost line from 13-16, Ford used a slot at the top of the divider between these bores (1-2, 2-3 and 3-4) cast into the top of the block but the passage get blocked over time by coolant degredation and or debris leading to a hot spot and point failure of the head gasket (again, often at #3 cylinder.) The design was apparently changed by engineers for the 2017 MY so 17-20 MY vehicles may be less prone to the issue. The alteration involved using diagonally drilled holes in the dividers between cylinders (#2 and 3) which increases coolant flow and eliminates the hot spots that lead to head warping, head gasket failure and coolant loss.
Ironically while the EcoBoost family is based loosely on the previous inline Duratec family of non turbo engines, few of them showed this flaw meaning Ford for some reason went with this slot idea over drilled passages when the conversion to the EcoBoost family was made around 2011. Ostensibly it might have been a cost saving measure (a slot in the top of the block between cylinders is likely cheaper to machine than drilling diagonal holes in every block) so the bean counters likely struck a blow here and messed up what was a pretty good product. Ford Tech Makuloco in his video notes this happens on higher mileage EB mills somewhere around the 70-100k range and if it gets catastrophic the engine is a goner. The advice it to watch coolant vigilantly and if the motor starts gulping it and you get misfire codes on the OBD it’s probably bad news. My wifes 16 Fusion has the 1.5 EB. I love the car. But at almost 70k, thinking it’s time to trade it out lest the motor goes in the next 20k miles due to this condition. One word of advice would be to drain and flush the cooling system earlier than the owners manual says (I think the service is at 90K) and try to remove anything floating around in there that may block these slots. And always use the Ford spec coolant because it has the right chemistry and additives for the alloys in these engines.