Amalgam fillings are bad if you want to keep your filled teeth for decades, they contract and expant with temperature and may crack themselves, or worse, crack your tooth. Composite are better because they bond with the tooth better and won't crack it, also they need less excavated room to be put in, amalgam fillings need to drill more of the tooth in order to be stable.

In general getting a filling isn't good, it may destabilize the tooth and make it easier to crack, but you have to get it or you'll need a root canal. Now for the long term ramifications for the tooth, my grandma has an amalgam filling that lasted her from her 30s to her 80s, I have no idea what the condition of the tooth is now, but she doesn't complain so it maybe good.