Well to be clear, the guide (since you didn't read it) covers multiple game elements. These include stealth, hiding, light and darkness, invisibility and surprise. You'll forgive me if I forgot something off hand. Furthermore, the guide explains the rules of the game specifically, I'm using a houserule to solve problems that the current system has created when it comes to an archtype feature built around obtaining surprise (which is essentially completely DM fiat and is super easy to control). I just don't want to allow surprise and then after initiative is rolled the assassin learns that he won't get to benefit from his feature because he didn't win initiative and I also don't want to allow surprise all the time to increase the chances of the assassin getting to benefit from it.
So since you immediately dismissed the houserule without even bothering to ask questions about it, I'll give a simple example of how the houserule works in practice:
Bobo the assassin is hiding in darkness. He sees his target getting close, who doesn't notice bobo thanks to his stealth vs passive perception roll which was high enough, and fires his crossbow causing him to lose hidden as a result. At this point, I fully resolve the attack made by bobo. Once the attack is resolved, I look and see who is surprised (if any). Anyone who did not notice bobo as a threat is now surprised. This includes his target if it's still alive. I then have everyone roll initiative and once that's done I follow the initiative order. If bobo's initiative is higher than his target, he can decide to attack his target again with any related benefit for his target being surprised. Then the next person goes and so on. Anyone surprised is surprised until the end of their first turn, after which they are no longer surprised and may take a reaction as normal. And so on.
So the scenario is simple, but it required multiple hoops by the participants to even allow for the "catastrophic" stuff you lament yourself about:
- Surprise was allowed by the DM. Let's face it, surprise simply doesn't happen at the table without DM approval. The entire scenario requires the DM to allow it to happen every step of the way. So by allowing an assassin to get surprise, you really shouldn't get to complain afterwards.
- Stealth vs passive perception had to be successful. If bobo had not managed to be hidden from his target, it would have noticed bobo and surprise could have been lost as a result based on DM fiat. This is because who an NPC considers a threat is up to the DM. Likewise, based on the description and story telling of the DM, the PC will decide who is a threat and who is not.
- Multiple attacks had to hit.
- initiative had to be high enough to beat the target's result so that you can benefit from the feature in question.
- Insert item here because basically the scenario could have multiple other hoops inserted or complications or consequences that I don't list here.
The reality is that you're doing what's called a knee jerk reaction. It's ok, it happens, but that doesn't mean that my suggested houserule is catastrophically wrong as you put it. If you want to discuss this further, feel free to suggest a scenario and we can discuss it so you can put your catastrophic fears to rest over the well being of my NPC's.
Lastly, I don't implement houserules without first discussing them openly with the group and voting if the houserule is used or not. I'm just explaining that for our group, the houserule has worked very well to remove problems we encountered with the current system as it stands and to make the assassin feature less rocket tag and more consistent across the board. And yes, it worked both ways and sometimes an NPC did get to have the triggering attack to start combat.
An example anecdote would be that the advisor to the king decided to murder the king at the ball. By the rules, I would of had to interrupt the party in the middle of the ball, ask for initiative for no reason (since nothing had happened yet that the party was aware of), roll initiative, win initiative and if I hadn't wait around for the advisor's turn to come up while the player's got even more confused, attack the king, and so on. VS with the houserule, the advisor kills the king out of nowhere, everyone is surprised, he runs off as people panic and the guards fumble to grab weapons and the encounter starts with the advisor running or whatever. I mean it's so much more fluid and immersive for everyone isn't it? feedback welcome of course.