Ah, you must either be new to the community or just young. This is my (non) issue with discussions like this and I was surprised it took this many posts to find one like this..... but they always happen eventually.
Thunderhead is far from unrated, it's just been replaced. When it was new, it was in everyone's top, it won the golden ticket award for best wood coaster, twice...even beating out Voyage, it's opening year. So it definitely had it's moment in the sun and was for sure talked about A LOT.
Coasters like Raptor (CP), Expedition GeForce, Balder, and Superman (NE) tend to come up a lot in topics like this too, also being rides that topped the charts at one point or another back in their prime. So maybe not under rated just no longer in the spotlight and I'm sure we'll see it with rides that's have opened in the last few years, eventually. Like when was the mast time people talked about Iron Rattler or New Texas Giant?