Just to note, I tried "Good thrones are made of iron" and it's not accepted.
Remember that Present tense for HV is not for general truths and facts (they accept answers like "men love big women" but that's not how we'd convey the same idea as in English). It is used simply for something that is happening right now but can stop being true at any point in the future. So it's something like "In my opinion that specific iron throne is good/useful (but one never knows what tomorrow holds)", rather than "Any/all iron thrones are good". We would use the Aorist for that (which btw we must learn independently since Duo doesn't have it available yet), like we do with "Valar Morghulis". More info at Dothraki wiki's "High Valyrian Verb Conjugation" page.