That there are differences doesn’t mean there aren’t also similarities. I mean, in your response, you point out how board games and sports are similar. Someone could claim that’s a false equivalence.
They are similar in that their rules contain the entire game. RPGs don't do that, at least D&D is far from contained in such a manner.
The truth is that they are all games. And games have rules and processes that make them function. Those rules and processes shape the experience of play… they help shape what’s considered fair or unfair. They simply do… it’s obvious.
The truth is, RPGs are not anywhere close to being contained by their rules like sports and board games are. They simply are not... it's obvious.
Some RPG rules are about fairness. Others have nothing to do with fairness. And a lot of the game doesn't have a rule that fits the situation, so the DM needs come up with a ruling/house rule. That last situation doesn't really come up in board games or sports games more than once in two or three blue moons.
What’s happening is that there are two types of unfair being discussed. There’s the unfair where something is clearly in violation of the rules. Let’s say like stacking bonuses that aren’t meant to stack, or altering/lying about a die roll. Unfair… no doubt about it.
Really? Let's say the DM and players all get to stack the same bonuses that aren't meant to stack. That's fair. Not the rules, but still fair.
Then there’s the unfair of the type you’re hinting at. Where it’s not a violation of the rules, but seems like a violation of some kind of social expectation. Something like a GM setting a DC higher than they should, or bringing in an adversary who is way beyond the capability of the PCs, and similar.
That's the only real fairness that there is. Any rules that touch on unfairness are ones that touch the social contract. Like no cheating.
This is the kind of unfair that can be incredibly hard to agree on. Sure, most folks would agree on extreme examples, and there will be times where folks generally agree. But the way it works… in this case the Alarm spell… it invites problems.
I mean… every DM can come up with an enemy that can bypass the alarm spell. It’s trivially easy to do so and then to simply cite (or make up) reasons that their decisions “make sense” and then justifying it all as “fun”.
The Alarm spell is rife with points where this can happen.
Not really. The Alarm spell isn't as rife with points as the OP would suggest.
"* Does the player's character have an uninterrupted minute of time to cast the spell?"
There will be somewhere close to zero times that this ever is an issue. If there are so many monsters that close to you, you probably aren't stopping for a minute to cast the spell. Outside of there being monsters everywhere around you, there the minute casting time just won't be an issue.
"* Does any potential intruder come within 8 hours, or do they turn up (say) 8 hours and 5 minutes after the spell was cast?"
It takes a rare corner case for this to ever be an issue. DMs aren't going to sit there and just have monsters show up right after the spell ends. And you know what? So what if the DM does. It lasts 8 hours so that the party can long rest and 8 hours and 5 minutes later, the long rest has happened and the encounters hits a fresh party that is awake for it already. This is the second non-issue brought up as a "point where it can happen."
"* Does a potential intruder come within the warded area, or open the warded portal? Or do they sneak around the warded portal, or inspect/attack from outside the area?"
This one is more common than the first point above, but still very rare. The overwhelming majority of encounters will enter the warded area, but a few might inspect or attack from outside if they typically are a ranged attacker. Still not an issue, though.
"* If the caster (and friends) are asleep, and are woken by this spell, how much can the intruder accomplish while they rouse themselves?"
They get to roll initiative since they are not surprised, which means they get their full round of stuff to do. They may or may not be wearing armor, and they may need to spend half their move to stand up, but they can do anything they would normally do. This is also a non-issue.