Since you pick 3 ethoses (or possibly one twice), you end up with 8*8*7 ways to play, instead of one if there were no restrictions.
I have as much ways to play as I want, as game allowing me. If player don't want to play everything but one thing, you can't make him just because you put any other options there. It's not infamious North Korea CK2 playstyle - because it is actually unintended way to play a game, it's screwing the system; but we're speaking about playstyles that ARE allowed for game. If there are 448 ways to play, I would have 448 ways to play with or without restrictions, but without restrictions you can select between them freely, not being forced to one you selected on the start, without actual knowing galaxy setup. Oh. "Yeah, you can change your ethos to meet situation, but if you can make it often it will be same with disabling restrictions, so it should be rare decision."
You see, when you say "we need hard blocks to make us play different playstyles", you're actually say "we have one "press-WIN-button playstyle, so we need to block it somehow".
The 'Xenophiles can't invade primitive planets' is a *great* example! It's a lot different to be enlightening primitives and getting protectorates that you eventually integrate then it is to just invade primitives and treat them like especially easy to colonize planets. Because of the restriction, you get different gameplay. It was a really good decision.
That's where we do disagree then. I'm sure it's a awful arbitrary (xenophiles are not pacifist, right?) decision that just locking gameplay options.
If invading planets are inherintly better that protectorates, so player would always annex primitives if he haven't roleplay reasons not to do so, it's not means somebody should be arbitrary locked to do so. It means designers should make having protectorates better or annexing primitives worse. If invading planets are not inherintly better, it's safe to allow players to decide what's fit them better, and you'll have different playstyles automatically.
I believe it's a very major point for me, so sorry - I'm trying to be clear. I believe that to create alternate playstyles you need to create real alternatives, not create classes. With classes you'll just pick a class you're comfortable to play and never will came out, because why would you? and some classes, until you can do perfect balance, will be better then another. Random example - you will always put an energy-giving building in the tile with resourse until you have real reason to do another way, and that will create similiar gameplay - and it will very bad decision just to say "you're collectivist, so you're screwed. You can't put anything but farm here. We did it to make gameplay different".
If you have different options, you'll have a choice, in-game choice, not some choice you did one time when created race. You can actually choose - "so, what's better for me now - is it psi ascension or robotic one? should I invade this primitive or put listening post there? should I befriend all that guys around or conquest them?" If that choices are balanced, playstyles would be different - I prefer diplomacy, my friend prefer conquest, that's ok, we don't need locked classes for it.
But ethos, your governmental ethos and your factions,
can play a big role here. They
can give you weights to do a choice. "Yeah. I can actually invade a planet, with this benefits and this drawbacks, or I can create a protectorate here (same - benefits and drawbacks). That's a difficult decision. But I have a lot of pacifists in my empire who won't approve using military force. So if I select invasion, I'll take big influence, production and unity hit. So I better take protectorate option!" Or you can say: "Well, yeah, it will hit me. But it's strategical planet I need, with a specie I want
now. So I'll invade it and take this hit, because benefits are better as drawbacks".
One can say I'm offering classes as well. I'm actually prefer to think about it as about "affinities". You can be flexible.
And flexibility in my opinion IS what's you need to differentiate gameplay.