The US says that the new cluster bombs are much more reliable

The US says a lot of things that are not true. They also said agent orange used in Vietnam didn't have lasting health effects but that is a horrifying lie. Children are still being born with birth defects to this day in Vietnam because of agent orange. In no way should the US be trusted with what's "safe/reliable". Especially in wartime, when the US and its weapons manufacturers stand to benefit from the sale of these weapons.

Ukraine should be allowed to decide how best to defend themselves.

No. As nations we've decided what weapons are inhumane and cannot be used. For instance they cannot use chlorine gas, no matter how effective that might be at clearing trenches