Which meditation is less likely to have negative effects or be practiced wrong?

I've heard a meditation teacher mention that samatha is less likely to be done wrong while using a visual object. I've also read about how mindfulness meditation may sometimes lead to negative effects in the practitioner. What is the type of meditation that is least likely to cause such possible negative effects, or to be practised incorrectly? Am I right in thinking compassion meditation is more accessible and less likely to cause harm?