I think a person might get a bit paranoid if they're unsure about how well they constructed the watercooling system.
I would (and do) put my best effort into making sure the WC system is put together in what I think is a good solid way. To me 'common sense' plays the most part of it, like not taking risks on the hardware I use (buy good strong components), and having the componentry assembled sturdily paying particular attention to how you secure the tubing on component parts. I mean, for example, ensure tubing is solidly connected, but at the same time don't overtighten things through 'fear' as doing this could effectively increase the chance of problems due to deformation (damage) of the tubing/barbs themselves.
As for the potential of pump failure, sure it could happen at any time irrespective to what pump it is, so if possible look for ways to have the PC monitor temperatures and perform a shutdown as a contingency plan for this happening. I think a lot of system BIOS's nowadays have the ability to specify 'emergency shutdown temperatures' by allowing you to set a temperature value, so definatly look into this. If you do this you might want to test that it actually works by firing up the computer to desktop and leaving it without the pump powered to simulate a pump failure. If you use a program like Motherboard Monitor (MBM) you should be able to watch the temperatures rise steadily up to the point where the system BIOS shutdown value is met at which point the PC should immediately shutdown. I think it'd be foolish to not going about testing in this way personally.
Other concerns. Personally I'd avoid the use of cheap acrylic reservoirs and similar like the plague. Although I've never used them I've read too many horror stories about the seals failing on them, and cracks appearing, etc, to ever feel the desire to use one myself.
Read up about 'galvanic corrosion' sometime particularly if your WC system is running mixed metals (aluminium and copper in particular). Make sure you use an appropriate water solution (distilled water & automotive antifreeze is common) to stave off long-term corrosion.
Keep an eye on water levels over time to see how fast they drop. Some systems seem to lose their water gradually over time due to microscopic leaks and such. Having an idea about how fast this occurs on your own system should allow you to learn how frequently you might need to top up the fluid (I top mine up maybe every few months with a few teaspoons worth of water, but all systems are different...).
Anyway, basically just make sure everything is good and tight but not overtight, choose strong components, build it well, setup temperature monitoring in BIOS or other to protect the system if something goes bad, and observe and learn how your system runs over time so you become familiar (and hopefully confident) with it.
No expert here but I built my own first WC about 15 months ago now and it's been running perfectly 24/7 (turning my PC off is a rare event!) all that time. I just check it periodically (water level, etc) and drain/refill the system once in a while (maybe every 5 months to reduce the likelyhood of growth/corrosion), oh, and dust off the rad every now and then.
hth.