While there are a handful of ultra rare mobs, Nytami and Mad Hogger seriously make me hate this game. I never had trouble with any rares when compared to those two. I personally like hunting rare mobs and like it there are are things in the game that require some serious work to accomplish, but I also don't think any rare mobs should be on timers so they only appear once in days. That means you either don't sleep and spend every moment camping a mob, which isn't realistic for most people with jobs, or run by every once in a while and hope you win the lottery. Thats all wonderful, but when you get down to the last few you need it starts to really become unpleasant.
As for mobs being broken, I think its just easy for us to think that because it becomes so ridiculous when a rare has not been seen in days. Its more likely that's just the way it is, or that the mob did spawn the 1 hour out of 24 in a day that you were not there. Its possible something doesn't work right, but its not really anything we can prove, and when GM's are contacted they just tell you its working fine.