All pumps 'burnout' eventually, either mechanically, chemically, or biologically. Lotta ways to die.
Sometimes, they up and die early - chalk it up to bad luck, RMA if possible, and try again.
The 360 and 420mm AIOs can limit mounting position and future chassis options. They are provided the same size tubes as the smaller units, and combined with the longer radiator, can become more of an inconvenience than a benefit.
Front mounting an AIO with the radiator inlet and outlet towards the top leads to a dripping or running water noise after an indefinite period of time(a few years or so). It's caused by the fluid level inevitably decreasing and the larger air pocket collecting at the inlet side of the radiator.
If your previous AIO was a Kraken M22 or one of the popular(only due to the pricing) Msi Coreliquid Rs: those units have their pumps in the radiator. This design changed what mounting options could be used. Mounting these at the top, or the front with rad inlet + outlet towards the top can see short runs from them, as the user risked exposing the pumps to air. Also, the Coreliquids seem to have a QC problem, 'cause they seem to fail fast even when mounted correctly!
Fractal Design recently launched their Lumen series - looks to be the same Pump In Rad design. Will have to wait and see how well those hold up...
CM C700P: That's a big one, at least on the outside.
Pictures online suggest it's not as deep(horizontally) as the 7000D is, as I can see that front mounting AIOs is possible, but you're still stuck with mounting the rad with inlet+outlet towards the top. The different layouts don't appear to help:
Inverted: One of your AIOs is going to be mounted at the bottom. Pump above rad is a no-no; you won't be keeping it for long...
Chimney: Intake air from the bottom, and everything goes up and out the top. One AIO will mount at the top, and neither can go at the bottom if you want it to last. This means one will still be put at the front as intake... Hey, that's no different from -
Regular: The usual.
Enthoo 719: No, I didn't. I've put it in a corner for now until the next gpu upgrade which I will just custom cool that part.
"The GPU AIO outputs a lot of heat, would that be a problem with using it as intake? Or as long as I have enough airflow it will move through without causing too many issues?"
No, it is not bad, per se, but it's no different from how the vast majority of systems out there are already set up, with the gpu dumping it's waste heat in the chassis and some, or most of it passing through the cpu cooler.
Between the cpu and gpu, the latter usually pumps out the most heat. With how power efficient cpus are today, the opposite is not as common.
-Gpu heat passing through cpu cooler.
-Cpu heat passing through gpu cooler.
Some air is still better than no air, but the 1st one is a less efficient use of it.
Ehh, I suppose it's all moot, when the PC ultimately warms up your room either way. Chalk it up to me nitpicking.