I guess it's mainly because the process of becoming a spartan seems trivialized with them.
This is by and large due to the community's false perception of them, because it is not, at all
, trivial to become a Spartan-IV. Safer? Yes, but there is still a screening process, as well as weeks-to-months of intense surgery and recovery time needed. There is also a grueling training regiment that is undergone to push the Spartans further than they've ever been. It's covered in multiple sources that it isn't as simple as just getting pricked with a few needles and BAM! done. Spartan made. It's a whole ordeal.
The augmentation process is a lot safer than it used to be,
As stated above; yes, safer, but it still contains its own risks, such as the Spartan-IVs needing to be "tuned-up" as it's referred, to ensure they don't have deterioration with their augments. They're only bested by the Gamma S-IIIs for needed monitoring of their health.
I also don't know why safer augmentations are a ding against the S-IVs: The S-III's augments were also safe. The maiming, disfigurement, and violent death incurred by augmentations was really only an S-II thing.
and the back stories are a lot less tragic than spartan II's.
Are they less? I find that "kidnapped at 6 and forced to become a child soldier" can also be matched with "watched home planet burned, constantly on the run from the Covenant, had family killed, fought desperately during the war to survive" for tragedy.
I really think the community refuses to believe
that the S-IVs can have a tragic backstory rather than it be innately true.
To be a spartan meant to single-handedly win a battle, to blow up halo rings, stop the flood while fostering relations with a race that you were once at war with for years.
I mean, only Chief did that. I'm not sure why that's the certification that's needed to hold the title Spartan. They can
do that, doesn't mean they have
to do it.
Now S 5's are in fire teams that can barly hold of Prometheans.
Some context should be kept in mind, as the Prometheans weren't an enemy encountered before. Even Chief struggled when initially encountering them.
It also appears that by the time of Halo 5 the Spartans--and UNSC--are more versed in fighting them: Osiris didn't seem to have much trouble throughout their escapade.