I don' think that the mark of a "great" or even a "good" game, any game, any format is... "I can't wait to house rule it to make it work." If I buy a game, I want it working out of the box. I don't want to have to wait for a patch, a fix, late-post-beta testing, or a Day-0-fix.
If I plunk down cash for RMU, I don't want to house rule it before I can play it.
Obviously just my 2c on the topic. I can see later on down the road, something's not quite right or it doesn't fit well with your group and you make a small house rule, but it shouldn't be a day-0 house rule.
After 30+ years of RM2, we have simplified initiative to something I don't think appears in any Companion (House Rule 1), and for Hobbies, we allow players to cut the DP cost of two digits for a skill or skills (House Rule 2). Since we start all players at level 5, that method makes a more balanced PC rather than "4 ranks and 2 ranks" in skills and due to the number of optional skills available from all the Companions, the lower cost in DPs allows players to grab some of those new skills.
Starting players at Level 5 isn't really a house rule, we just like to start at a slightly higher PC level. The weekend campaign I'm running, I'm starting players at level 1 on Friday, but there will be 4-5 one-offs that will have the players all at level 5 by Saturday morning to get into the Grand Campaign.
30+ years RM2 ---> Two (arguably three) house rules
We limit all magic to only Elemental Companion. Could that be a considered a house rule or it's just the magic that exists in our gaming world?