In my experience, cloister tiles in Carcassonne are often "too lucky". If you draw a cloister tile in the beginning of the game, it will typically still require an investment of quite a bit of "meeple" time to obtain the full 9 points, which makes it a fair trade-off. However, after about half of the game, it's relatively likely that you can "parachute" a cloister tile in some spot and get 8 or 9 points immediately. This adds a lot of randomness to the game.
What house rules work well to diminish this effect?