Android is like a box of chocolates. You never know what you are going to get.
Each manufacturer takes the Android OS and tweaks it. Two different phone manufacturers using the same OS will have phones that run differently. Even though the two phones are using the same OS, they will require a different update from the particular manufacturer. That is one reason why even though ICS has been out for about 5 months, only a handful of phones have been updated to ics. Some more technically savvy users will root to take update matters into their own hands.
Also, an app may work well on one phone, but not very well on a different phone even though the OS is the same. That is why when an app gets updated, you may get reviews by certain phone owners who will say that the app stopped working (well) on their phone. What I have learned to do is two things when it comes to updating apps for my phone:
1. Wait a couple of days after the app comes out and read the reviews on Market to see what other users have to say about the new app. If the reviews are bad, it may be better to just not update at all.
2. If I do decide to update an app, I always use an app backup manager eg. ES Explorerer to make a backup of the existing app before updating. If the new version is buggy or contains undesirable new features (eg. ads, laggy, worse UI etc. ), I can usually revert to an earlier version.