One of the most important reasons for Ukrainians being able to defend themselves against the Russian military is the weapons sent by various NATO / EU countries.
However, it seems that there is quite a reluctance on sending more powerful weapons compared to what would be possible and the Russians are making progress in Eastern Ukraine.
For example, the US is sending advanced longer-range rocket systems to Ukraine, but this happens quite late.
This delay in sending powerful weapons that would have helped the Ukrainian army push back the Russians faster has arguably led to more Ukrainian cities being almost completely destroyed.
I know that various countries might have various reasons, so I would exclude from this question the countries that still significantly depend on Russian oil and/or gas (e.g. Germany, Hungary).
What reasons have the other countries given for not sending more powerful weapons or what risks are attached with sending these weapons?