I understand your disappointment, but think you are focusing your entire perception of the program on (basically) one benefit - suite upgrades. And suite upgrades at some of the most popular properties in the chain!
1) others are correct, points posting can be an issue. But it is an issue I have at Marriott and Hilton - so IMO that is just a basic hotel/program flaw in all chains. I am surprised you have never had an issue with Hilton...but great for you!
2) if you are asking for a suite upgrade during a conference, chances are the suites have been booked by paying customers. there is nothing to intercede on there - sold out is sold out.
3) all suites are not created equal. Suite Upgrades never qualify for Presidential Suites, etc..... so although there may be many rooms (even suites) available, if it is of a type that does not qualify, again - nothing Diamond desk (or anyone else) can do.
As you admit, you "decided to go for a Diamond trial, since I have a conference coming up at a Hyatt." Sometimes it can be disappointing when one joins a program for a specific reason, rather than the program (and properties!) as a whole.
I would suggest you give Hyatt service, stellar properties, club visits, etc a little more time. Then you can make a decision based on multiple aspects of being a Diamond, rather than just suite availability.