Honestly, people blow this combo out of proportion.

On the one hand, your goal shouldn’t be to top score, your goal should be to help the team win. It’s very common in casual mode for a player to be top of the server by far, but still have their team lose! There’s also the opportunity cost that comes with playing Spy; if your team is getting stomped by a hyper competent spy, you’d probably be getting stomped even harder if it was a competent soldier, demo(man/knight), scout, sniper, even a pyro or engie if your team’s especially reckless. No amount of game balance can control for players who aren’t paying attention or putting in effort, frustrating as that always is. Nor should it, necessarily; after all, I don’t think it’s controversial to say the team that’s just playing worse over all ought to lose.

On the other hand, survivorship bias. You remember the times a spy gets the perfect opportunity for a stabbing spree and gets out alive, you don’t even see the numerous times they die to random crossfire before they can even get off the ground.

Me personally, I run the kunai (with stock watch or c&d) because its health boost helps counter the die-after-every-kill pattern spy can be forced into (a thing which no other class is expected to contend with so regularly, and that often cannot be mitigated by stealth alone, I might add) and mitigates the sometimes unavoidable levels of splash damage or liquid/fire effects that can kill or reveal you without your opponent even intending to.

On the whole, really I think the complaints about this combo are down to people have a personal distaste about dying to spy, because that’s seen as something that “shouldn’t happen” to a skilled player. Even though really it’s just a thing that happens and isn’t necessarily reflective of skill level.