I have never had a problem with an AIO failing on me or leaking and have used several different companies over about 11 years. If the pump did fail the CPU heat sensor should shut down before the CPU cooks itself also older CPU's didn't really have that feature. I would say I feel that it's about as safe as using air cooling really.
 
As for air cooling I kind of wonder if the coolers on the motherboard itself would block airflow. It seems like it would at least be blocking some of it.

I have a Noctua U12A mounted on my z390 dark. The heat pipes just rub against the rams. Be careful of their heat spreader height though. I'll post photos when I take some later tonight.

Ok  I officially give up on the air cooler and will reluctantly pass and hope for no leaks since I can't afford to replace anything if that happens. 
Here's the build plan:
 
Intel Core i9-9900K Coffee Lake 8-Core/16-Thread Processor Socket LGA 1151, 3.6 GHz Base/ 5.0 GHz Max Turbo Frequency 95W Gen9       
Case:   Core V71 Tempered Glass Edition   CA-1B6-00F1WN-04
EVGA Z390 DARK                                                                                          
G.SKILL Trident Z RGB Series 32GB (2x 16GB) 3466MHz C16                      
EVGA Super NOVA 1000 G2 1000W 80Plus Gold (20-G2-1000-XR)                 
EVGA CLC 280mm All-In-One RGB LED CPU Liquid Cooler 
Thermaltake Gaming PCI-E 3.0 X16 Riser Cable (AC-053-CN1OTN-C1) 
WD Black PCIe Gen3 x4 NVMe M.2 2280 1TB Read:3400 Mb/s, Write: 2800MB/s SSD (WDS100T2X0C)                   
Arctic Silver 5 (3.5g) High-Density Silver Thermal Compound                        
WIN 10 Pro 64 bit OEM DVD                                                                                       
Corsair LL Series LL140 RGB 140mm Dual Light Loop RGB Fan  
    
Just using my old videocard for now until I can afford to get a better one, computer first than videocard     
Couldn't find a list of compatible memory here but Gskill lists this memory as compatible for the Dark  
I am a little worried that if I mount this CPU Cooler at the top of the case (Only place I can put it) being wider than a 360 Rad with 140mm fans on it, it might get too close to the memory.  Probably one of those things you have to actually try it to know it'll work for sure, I know memory has to be in before I can mount the cooler on the top of the case of course.
 

I use a Noctua D-15 over G.skill Trident Royal ram. I use a single Noctua iPPC 140mm 3k rpm fan in the center. Fan orientation will have to be facing up. You can possibly fit another fan on the underside but you will find reduced thermals being right up against the backplate of your GPU. I only strap an additional fan on the topside when benchmarking, but you will need a very large case as the height of the fan is now slightly taller than the motherboard itself. Also I tried the Dark Rock Pro 4, it does NOT fit. You will have partial coverage of the IHS. I have not used any other air coolers with the z390 Dark. Hope this helps.