I am mostly refering to the famous female-fronted bands (Nightwish, Epica, Within Temptation etc) , as well as the newer, lesser known bands that aspire to be clones of the aformentioned. I think this for 3 reasons:

1. Pop song structures. The vast majority of the songs in this category follow the formulaic pop structure of verse-chorus-verse-chorus-bridge-chorus, with little to no deviation. Sure , there are some longer songs that break this formula (Ghost love score, Kingdom of heaven etc), but these songs are more rare (and you can find some verse-chorus repetiton even on these).
2. Vocals are the main driving force of the songs, which is a very pop characteristic. This includes catchy vocal hooks and sing along choruses. There is also too much fixation on the female vocalists (which one is better, who are the top 10 in the genre, which era of "insert band name here" vocalist is better etc...). Paying too much attention to the vocals and neglecting everything else is what pop is all about. The focus of metal is the guitar riffs and the focus of Classical/symphonic music is the orchestration and not the vocals (with the exception of Opera). Most classical symphonies and concertos don't even have a choir. I'm not against vocals, but they shouldn't be the main priority.
3. The orchestral elements serve more as ornaments/layering ,providing an epic atmosphere, and can rarely hold on their own as well-written compositions ( Of course there are exceptions to this, mainly some of Tuomas' work). Even when the orchestral compositions are more extended, they are too simplistic and monotonous , with typical chord progressions and predictable, Hollywood-ish crescendi ,just to provide this extra "bombastic feeling". They lack any depth, complexity and nuance that characterizes classical music. ( On the contrary ,some of the best bands from that perspective , like Therion and Septic flesh, use the orchestra as the main driving force of the song . Examples : Therion-Via nocturna, Septic flesh- Oceans of grey, Dimmu Borgir- Progenies of the great Apocalypse ) . This would be less of an issue if at least the metal part was interesting, but unfortunately most of the riffing is simple chugging or power chord progressions (Epica have some cool riffs, but ultimately they fall short of the riffs of a typical Death/Thrash band).

This is my take of the music of the more mainstream bands in the genre. If you are aware of any bands that don't follow the aformentioned tropes , please do share. It's not my intention to shit on the genre, it's just that i am quite dissapointed. I started listening to symphonic metal expecting some deep fusion of classical music and metal , combining the better nature of both, the energy of metal and complexity of classical. But what i got instead was, well, pop metal.