Acrylic is one of those mysterious materials in waterblocks, done right with normal safe fluids and it can work for years without any sort of problem. I have acrylic in my D-Tek Fuzion GFX 1 and Danger Den MCP chipset blocks that's I've used for a long time now without a hint of any problems. Then there are those myserious reservoir issues that don't at all appear to be associated with bolt or bending stresses. There is also a variety of acrylics and some are apparently better than others.which one you get with any particular block is anyones guess.

All we know is there have been some problems with some blocks and some reservoirs, but nothing particularly consistent. It's hard to say it's the design, it could very well be the fluids used and/or the particular type of acrylic. You can be 100% certain that a block or component made with acetal won't crack, you can't say the same thing about acrylic.

I haven't seen a block or reservoir made of polycarbonate that has cracked, but it also hasn't been using nearly as much. I wouldn't expect to see polycarbonate crack, it's many many time more flexible and resistant to tensile stresses. Not so sure about chemical resistance...water and PTnuke is a pretty safe bet though.