Let me preface this response with: There is no right way or wrong way for tubing other than how you run your tubing.
That said I find these two Cons to be the deciding factor in what I run.
Rigid Con: Replacing parts can and often do require one or two more tubes to be bent. This is specifically for MB, and GPU upgrades. In some respects you can avoid this with a couple of fittings added at key points but if you're a Bend Hound you would need to replace tubing.
Flex Con: 1st and Foremost is Plasticiser. There are few tubing manufacturers that don't use the stuff in vinyl flex tubing. Heat is what activates it and it WILL leech into coolant and cause us Water Cooling Enthusiasts headaches later on down the line after the initial setup process is complete. Flushing every component in the loop to get the stuff out of it. If you add dye to your mix it will color the stuff and you will see that not only will you have to flush one or more Radiators, you will have to clean any microfins cut into your blocks. Not only that but then you have to purchase more tubing to replace the original tubing. Some day they may come up with a way to connect all the fittings that don't require any tubing, but they don't so yeah.
The latter Con is why I chose to go with Acrylic. I won't ever purchase PETG because if I ever want to go with a Premix(EK or otherwise) it may contain Ethylene Glycol and that is a component of the PETG Formula. I don't feel like having my tubes melt from the inside out. No matter how slow a process it may be.
Of course, one could always go with Copper tubing and avoid it altogether. I just may do this in the Winter since this Summer has been fairly hot and Copper would add a little cooling performance. Probably not more than sub 1*c per tube but it does have surface contacted by coolant and with a little airflow should be good for that at least.
Well anydoo... the Enthusiast has to come to the determination, how much time, energy and money they wish to sink into something like fittings, tubing and the work that invariably comes with that choice. Personally I would rather(and have) go with rigid as I can change any tube and it only takes a few minutes, not included drain flush and fill time. Even with that it's still less time and energy than having to Flush and Fill a Flex tubing setup gunked with plasticizer.