I'm not attempting to make the case that every tech is entirely different in Ascendancy; there's certain things that simply have to exist, and certain hardcoded limits we have to work within (a lot of the more interesting techs have to be done with abilities tied to planets or structures which get unlocked by the tech and jury rigging a buff chain out of that for the desired effect, as opposed to being a direct effect of the tech itself)- each faction has to have some increase in income, some increase in culture, etc. We have a bigger difference in what those numbers for each stat mean to each faction, though. There is a perception I've seen from some people that the base game actually does different things, but that's simply a side effect of how the base tech tree is arranged- they have the same sort of branch in different places doing the same thing, whereas we've made the organization standardized so as to help people transition between factions (plus simplifying the icons to the type of stat that's being changed makes it seem more similar than it is). As far as defense (and diplomacy) goes, that's true although also simply a function of us having not reached the point where we're working on that yet, as of 1.0.
i'm not trying to make the argument that Sins is a bad game or that the factions in practice end up being homogeneous. i'm making the argument that the things that make the factions different (one of the most important aspects of a game like this) are sadly the results of some of the poorest decisions made in development. A lot of them just plain lack counter-play or have very little interaction from the player; some are downright frustrating. The links between the TEC and pirates have some interesting aspects (the alliance part, which led me to the strategy of hiding my flagship in pirate bases) but also had the random pirate fleet spawns in at the enemy planets. It's one of the key hooks of the faction, but you're doing literally nothing to play around that. You don't control the ships, you don't even have some sort of budget for attacks or targets; it's just an RNG mosquito-like pest (the thing where the militia don't attack you is cool, but in the context of the mod would mean things like Warlord bases and minor factions would also be aligned with you, which makes it unusable).
The Vasari node-bypassing effects of phase gates and superweapons are also problematic for some similar reasons; if it were just a way to travel quicker between your planets, that's mostly fine (and also something I've considered for the Empire of the Hand) but once it lets you bypass border worlds and attack wherever, whenever with very little risk because of how common the superweapons become, it's just pure frustration. No matter how well you plan or how much you invest in defense, if the Vasari jump their main fleet in the back of your Empire, you're probably gonna lose at least one planet for very little cost before they can either progress or jump back to safety. It becomes a race to see whose non-interactive mechanic such as this becomes more powerful sooner.
Essentially, the differences in Rebellion specifically come from Titans, those non-counter-playable or non-interactive mechanics, and the specific effects of their superweapon. Sure, it does end up requiring some different playstyles but it's ultimately, in my opinion, for the wrong reason, and the techs not related to that ultimately don't end up mattering. It's all about how soon you can do the uncounterable culture-bomb + carrier spam with the Advent, phase node with the Vasari, or be an annoying little jerk with the TEC (as ended up being my preference, with the Ragnarov titan). The Titans ultimately also are unique and provide a lot of the unique elements, but that's also because they're so central to the strategies you employ; ultimately, your fleets are going to have to be built around a titan and you typically can't hope to beat a strong titan without one of your own. This can have some cool and unique effects like the Railgun and the Maw, but it's also a central cause of the 'fleetballing' I've always hoped to reduce in Sins, which is why we're so reluctant to give Titans abilities that are too powerful.
Before we made Ascendancy, the team played a lot of base Sins amongst ourselves, and even talked about doing just a small base mod to ameliorate the parts of the game that we found obnoxious while we were getting to a point where we could actually play Ascendancy, and these are essentially the things we would talk about nerfing. Everyone would get pissed at me for rushing the Insurgents that I no longer had any influence over after clicking "research"; everyone would get annoyed with Ca3ba and Kalo for their shenanigans with Orkuluses (Orkuli?) and phase nodes that we saw coming literally the entire game but could ultimately do very little about until after it happened and hope our fleets could take out more of their planets before they took out all of ours; we'd all get annoyed at Code for his culture taking us out entirely on its own, passively, instead of being a tool that if used properly helped you accomplish other goals, all the while cannon shells flying from planet to plant every 3 seconds. And that's the thing; these are unique, but they're all frustrating and its not frustration because you're outplayed or caught off guard by some strategy, it's just "I know exactly what's coming and when, but ultimately none of us has control over it or can prevent it."
If you think about development of the game/mod like a maze, where the exit is having made something that's engaging and fun with unique factions, in some ways Sins may be closer to the actual exit than, us, but they've also done that by running into a few dead ends that ultimately render that progress kind of meaningless; where we may be slightly farther behind in some areas, we're trying to do it in a way that actually allows us to reach the goal. If for example, Dave feels that the base game has achieved those unique factions, he's entitled to that opinion, but in my opinion what they've achieved in terms of unique factions is for all the wrong reasons and ultimately detracts from the game and is a matter of flash rather than substance.