To MagnusPhil's point about groups mixing grots and marines, that is entirely beside the point. Even if the group was all one or the other, the system would have to be built to represent grots and marines. That is the issue.
You might notice that I prefaced my post by saying that Lynata's argument, which is what I commented on, was missing the point. Which, inherently, makes me "miss the point" as well.
I'm not saying that the system as-is would work for it. I'm saying that mixed groups isn't something that the system must cater to. Grots do not need to be as good as Space Marines.
That said, I don't necessarily think this is a problem in a d100 system, and I think I disagree that the system would need to be remade from the ground up. I've yet to see a compelling argument either way, and so I don't have a set opinion.
I don't think this is a separate argument at all. There is a difference between being "better" at something, and between completely dominating entire segments of gameplay.
And a unified system should at least attempt to offer all characters a chance to be useful. Like "Inquisitor" did. I see no need to artificially inflate the gap between characters more than absolutely necessary, especially when this hampers group synergy.Some characters have a certain "minimum of superiority" that comes essential with their background, and this needs to be catered to. Anything above this minimum, however, just means intentionally sabotaging everyone elses fun - especially when it can be argued that other characters should share this level of superiority. As previously hinted at, Space Marines do not have an implant that inherently makes them better shooters or swordfighters. They die to bolter rounds and even las weapons like everyone else - at least in GW's material. You just need to hit them a couple more times.
The different races are not so unbalanced as you seem to imply. A gun makes a wonderful "threat equalizer" in 40k, at least as long as you are not going FFG's route and give Astartes +1 gear because they are not yet awesome enough.And if you feel like pulling comparisons, perhaps look for better ones than Grots, who would have trouble keeping up with normal humans. Like, maybe look at mixed groups like they actually exist in GW's fluff. After all, isn't this what this is all about?
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Comparing lowly Grots to Space Marines was exactly the point - finding a huge divide. We could say Grots and Greater Daemons, if further hyperbole better conveys the idea.
The point is, a universal system should be just that: Universal. It should be a sandbox in which you can build whichever 40k RPG you want. I should be able to play a Grot's Journey to Nobdom, or I should be able to play out the internal power struggles of greater daemons.
Trying to make a party inherently balanced in such a system is impossible. Or at least it would suck so much flavour out of the world that there'd be no point.
The system should absolutely not "try and make an effort to make everyone feel useful". That's the GMs job. If you, as a GM, allow 1 player to be a Space Marine while everyone else is a Hive Dreg, you're a terrible GM. You can help this by giving the Dregs some starting XP (for which a guide in the core book would be very useful), or some other advantage - but saying that the system should reward weak races for being weak is a terrible idea.
There's two arguments now (which is why I said your point was a separate one):
1) Whether or not the d100 system is at all capable of representing the vast power scale in the 40k universe.
2) Whether or not the above would be fun .
I think the first question is interesting. The second one less so, at least until the first is answered.