The biggest issue is the wide range of new offenses it requires sites to remove and the creation of intermediary liability in multiple cases where what the bill wants you to do isn't clear at all. The rights it creates for UK citizens and the obligations it imposes on UK platforms are also contradictory: it gives UK citizens "freedom of expression", but at the same time requires sites to censor multiple categories of "harmful" expression that are so vaguely defined platforms will inevitably interpret in the most narrow and restrictive means possible to avoid incurring liability. Literally no one agrees what content online is "harmful", and any government attempt to regulate it is going to have massive splash damage.