I wouldn't say that they are good or worse... I mean some of the UC stuff isn't great either.
But, the thing with the Alternate Universe is that some can be VERY different than what "Gundam" originally is. Sticking with UC Gundam first will definitely give a better foundation to what Gundam is. Most of the AU's are very good, but tend to mix the way the series can be defined.
Like G Gundam can be considered basically Dragonball Z Gundam. It's heavily martial arts based and handles things VERY differently. Some people liked this, some hate it.
It really is better to get the grounding of what original Gundam is, THEN move into the alternate universes.
...then again, some of the AU's are my favorties... Like Gundam 00 and G Gundam.