I have used acrylic often in my blocks and rarely had an issue. Then moved to Lexan which is much better. There are ways of dealing with the micro-fractures but few take the time to do it. There is also ways of preventing cracks around screws but few seem to listen (one Eddy dude comes to mind).
The largest problem is uneducated end users that think they need 2000ft lbs of torque on each screw so water won't leak. And this is the main reason IMO a manufacture should use copper or brass tops on blocks. Delrin and the likes will work better then acrylic and lexan but I am still not 100% confident in the end users.
All the blocks I design are DIY but I try to think about "what if I sold these" when making them. Thats why all my recent blocks have copper tops with soldered on barbs. I was impressed when Swiftech used the same concept with their MCW6000 series blocks. Small things can come back to bite you in the ass as I am sure any manufacture with experience knows, yet they still take the short cuts that leads to the sore ass.