I've seen similar cases where insurers try to dodge cosmetic issues on metal roofs—it's pretty common. Metal roofs are tough, yeah, but the insurance game is tricky. Asphalt shingles might get approved quicker since damage is usually obvious and structural. With metal, insurers love the "cosmetic" excuse to avoid paying out. But like you said, if you come prepared with solid evidence from a neutral expert, insurers will usually cave eventually.
Still, I'd argue metal roofs are worth the hassle long-term. Had a homeowner last summer whose asphalt shingles got shredded in a hailstorm—insurance paid up quick, sure, but he had to replace the whole roof. Meanwhile, neighbor with a metal roof just had dents...annoying, but still fully functional. So even though the claim process can be a headache, metal roofs save you from repeated replacements every couple storms. Just my two cents...
