One side effect of house rules, in my opinion, is that they both tend to create unreasonable expectations across cultures, as mentioned above. But they also have two other side effects.
- They tend to become somewhat enshrined, even moreso than the RAW. Someone once upon a time created a house rule to deal with an issue. Nobody remembers what the issue was, half the time. The reasoning for the rule is rarely clearly explained. How it fixes a problem is left to vague interpretations. But nobody wants to remove it, even when it seems senselessly restrictive, because nobody wants to risk pulling the band-aid off of whatever bullet wound is covering it.
House Rules need to be written, IMO, like court opinions. You need to both announce the change to the rules, and explain the reasoning for it (which is an excellent use for MU Talk Pages). You need to offer some background to explain why this fixes a problem, so that (in the event someone has an idea that actually does it better) you can scrap the thing. Or, if it turns out you were just being needlessly reactionary after people review it, you can remove it and go back to basics.
- They tend to create serious confusion for Storytellers, which leads to less people telling stories. People don't like dealing with rules above and beyond the rules they're already learning for the game system. In games where you have multiple spheres, like WoD, this becomes rather quickly apparent as you have to memorize sometimes a whole supplement's worth of base rules to incorporate one character AND THEN go through and learn all the house rules for it.
It seriously restricts storyteller freedom, and staff will often go back through and review things, tap people on the shoulder, tell them why this story that two people ran essentially for their own fun doesn't work, etc, because once upon a time some staff member made a ruling (recorded or not) on some mechanic and now it's that way FOREVER. In some cases, it's justified, but in others, it makes the person who was trying to create plot even more gunshy. So you end up with no storytellers, because fuck that noise.
So house rules need to be implemented sparingly, if at all (I still feel that the best way to resolve these things is that the storyteller at the time makes a call on it -- subject to appeal to higher powers if it's just way too whacky -- and that's the way it goes). Things don't always work the exact same way, the exact same time, especially when you get into areas like magic. Wind blows a bullet off course by a fraction of an inch, the processor in a computer gets a sudden unexpected load, and the confluence of certain magical energies causes unique, interesting, and unpredictable effects. Storyteller fiat should overrule house rules, and even RAW when called for, but those things should not necessarily be enshrined in formal legalese within the game.
If you DO enshrine them into formal legalese, then please, offer some elucidation on why you think this is so important it has to apply to all people forever.