I think people are too biased against thargoid gameplay.
It is a unique space fight experience. The walls and the exclusivity of weapons are all a prerequisite to make it so unique.
Source: https://www.youtube.com/watch?v=8wPG8fDQknE
I mean, I've fought them. Scouts more than the big ones, and only a Cyclops, but I've done it and unlocked the weapons. I'm not interested in a boss fight, I want something that impacts my day to day gameplay in Elite. It's so disconnected and isolated, that even ignoring how annoying the Thargoids-only weapons thing is, the whole experience doesn't feel like it's part of the larger game. You have to force it to happen.
The challenge is good, even though I've never tried one above a Cyclops I've seen plenty of video, but it has nothing to do with regular gameplay. It may as well be a challenge scenario from the main menu. The overall experience itself didn't feel special (aside from how cool the Thargoids themselves are, I wanna stress that), it felt like a walled off experience that you gotta go through a lotta hassle to take part in.
That's one thing I feel like Frontier often trips over-- the difference between challenge and hassle. The Thargoids themselves are a challenge, getting to a point to deal with them isn't. It's just a hassle. And since it's not a part of the rest of the game, I feel little-to-no motivation to go change my loadout JUST for Thargoids, cutting off any natural flow from the rest of the game, for a boss fight.
It's just my opinion, but that's why I don't get a "wow this is so unique" from the hassle of getting the weapons (and I won't feel that if I synth ammo for the new MC, I'll only be annoyed... collecting Guardian materials is again a hassle and not a challenge). When I see a Thargoid, I
do go "whoa that's unique", because they're really cool creations. They did an awesome job with them on every level, visually and aurally alike. And they're appropriately deadly, and would remain so even if our normal weapons could be used on them. More importantly though, they shouldn't be walled off, they should be giving me a problem I am forced to deal with, but I can pretend they don't exist and it doesn't change anything.
I like to wander around, do different missions, fight a bit, smuggle, etc etc. There are things that can happen while I'm out doing this and the gameplay can flow together. If I am fighting Thargoids, I am ONLY fighting Thargoids due to the Thargoid-only loadout required, with the one extra thing that could happen is a CMDR defending the Thargoids attacks me and then I can't defend myself because of the Thargoid-only loadout, which is also annoying. Them being walled off and requiring weapons that only work on them simply makes me not bother.
If I am amblin' and ramblin' around and a Thargoid shows up to mess my ish up, GREAT! It'd still be annoying that my weapons can't hurt them, but it's at least creating a problem for me that I gotta solve somehow. It's throwing a wrench in whatever plans I had that day, and I'd LOVE it. Scouts intercept me when I'm trying to go refuel? Awesome! Interceptor shows up? Oh no, gotta run! But it doesn't work like that, and that is really disappointing to me. Boss fights that I gotta go out of my way to experience just don't interest me, I guess, but I am happy if someone else enjoys them.
This "synth AX ammo" business just underlined for me how annoying the Thargoid experience is, but hopefully the Thargoids can join the rest of the game and not be off in its own corner away from everything else.