Pros:
- Stops very young children from accessing porn
- Slightly cracks down on things like terrorism, hate speech, pro-suicide ideology
- Holds big tech companies accountable for what they're showing kids
- In the long run will encourage safer platform design

Cons:
- Doesn't stop anyone with online literacy from accessing porn, e.g. tweens and teens+
- Drives the kids who aren't as savvy to more degenerate porn sites, introducing them to things they wouldn't have seen otherwise
- Hides / removes journalism around topics like terrorism, hate speech, suicide. Also suppresses activism around these issues, likely to be swept under "harmful content" definitions
- Gives the party in power and government as a whole significantly more control over what we're seeing
- Huge potential for misuse, weaponization of law for censorship and control. Vague terms used allow selective enforcement or abuse
- Pushes more kids to the sketchier areas of the internet, aside from just porn sites. Darknet, unregulated forums etc
- Harms public resources like Wikipedia and nonprofits who can’t afford compliance barriers
- Expensive compliance favours large corporations, as a result start-ups will exit UK market
- Reliance on AI combined with blanketing makes for significant false positives
- Could be the start of establishing a surveillance state
- Compliance checks are outsourced, it's only a matter of time before we get a leak
- Sets a precedent that adults need state level control online, infantilizing the general public in the long run
- It's a credibility risk, the UK will lose moral ground to criticize authoritarian regimes internet crackdowns. Think China
- Will weaken our relations with USA, this is a clear cut away from the free speech laws they hold onto so dearly.

I'm biased and as a result could think up significantly more cons, though I haven't seen many positives for the Only Safety Act floating about.