I knew that some of their units had the pump in the radiator, which would allow for bottom mounting without issue.
Not true, actually.
For Pump-In-Radiator...
Top mounted rad: hands down the worst of the 5. You can kiss that pump goodbye within a year.
Front mounted rad, but inlet+outlet are at the top: ok, but will become noisier later on after a few years(+/-), forcing the user to either move it, or replace it, even though the unit still works.
Front mounted rad, but inlet+outlet are at the bottom: one of the 2 best ways to position these units in part, because the pump is offset and not in the center of the rad. The problem then, is radiator size and chassis compatibility.
Sideways mounted rad(think NR200P): right there with front mount, but bottom mounted in+out - it's great.
Bottom mounted rad: the pump is saved, but now the performance goes down the drain, as air begins having a party in the cpu block disrupting the transfer of heat from the cold plate to the medium(fluid)! The party only gets bigger over time.
For the more traditional Pump-In-Block...
Top mounted rad: awesome, but you may need to over-provision on size depending on the gpu.
Front mounted rad, but in+out are at the top: ok, but the noise later on.
Front mounted rad, but in+out are at the bottom: also great, but radiator size and chassis compatibility.
Sideways mounted rad: great as well.
Bottom mounted rad: bad. Air party in the pump.