I have a similar shelf (looks the same, purchased from Lowe's, except it 48" wide) and initially had the same hesitation to use it.
The individual shelves are compressed particle board, which is more water resistant, but as added precaution, I wrapped each board in several layers of heavy duty thick plastic sheeting. This has protected them from the standard spills and one occasion where the filter backed up and spilled a gallon or two of water (which pooled on the plastic).
So far I've got one level with 4 ten gallon tanks, and on another level I've got 3 ten gallon tanks, plus a number of smaller containers for shrimp, green water and baby fish. The stand is super stable -- with one caveat -- take care to distribute the weight of your tanks evenly on the shelves.
My advice is to not set it up the way I did initially. At first I had 2 ten gallon tanks per shelf -- long side forward. This caused the boards to bow slightly in the middle, so the tanks were not even. Now, I set the tanks so that they are short side forward (this is how I get 4 tens per shelf) and the weight is now distributed more or less evenly. It's easy to verify that the tanks are level, just check that the waterline is the same distance from the top at all the corners.
I initially started out slowly with loading this thing down with weight, I only had a few tanks, partially filled...I've since put most of my 10 gallon tanks on it, and they're all more or less filled with water. I no longer have any hesitation about using it, though if it helps you sleep better at night, there's no harm in using extra boards/lumber to fortify and protect.