Myth: it is loud in the rain
The barn-roof sound people remember comes from panels screwed to open purlins with nothing underneath. A house roof has a solid plywood deck, underlayment and an attic full of insulation between the metal and your ears. Rain on a residential metal roof sounds like rain on any other roof.
If anything, the extra underlayment layer used with metal makes it quieter than a bare tile roof.
Myth: you cannot walk on it
You can, carefully, the way you should walk any roof. Step on the flat of the panel near the fasteners or over the ribs where the manufacturer says, in soft shoes, on a dry day. What you should not do is walk it in the same boots and the same careless way that cracks tile and scuffs shingle.
I walk metal roofs for inspections all the time.
Myth: it attracts lightning
Lightning goes for the tallest object in the area regardless of what it is made of. A metal roof does not draw it any more than a tile roof does. If lightning does strike, a metal roof is non-combustible and conducts the energy to ground, which is safer than a strike on shingle.
In the inland valley lightning is rare anyway; wind and heat are the real weather.
Myth: it rusts
Bare steel rusts. Roofing steel is galvanized or aluminum-zinc coated and then painted with a factory finish, and the good ones carry long finish warranties. Aluminum and copper panels do not rust at all. Rust on a modern metal roof usually means a cut edge left unsealed or a mismatched fastener, both installation errors.
Choose a proper fastener and trim system and rust is not a concern.
Myth: it makes the house hotter
A dark metal roof absorbs heat, the same as a dark anything, but the panel is thin and sheds that heat quickly once the sun moves. A light or cool-pigment finish reflects most of it. Compared with a dark shingle roof, which soaks heat into the asphalt and radiates it into the attic all evening, metal is usually the cooler choice.
Ventilate the attic properly and either roof does better; metal just starts with an advantage.
One myth that is partly true
Metal is harder to repair than shingle in one sense: matching a panel color years later can be difficult, and a badly detailed seam is a real fault. Both are answered by a good installer and by keeping a few spare panels from the original run, which I always leave with the homeowner.