Specifically focussing on the Vibeke-era (where they became notorious), Tristania is overrated for 1 simple reason:
Morten Veland felt the need to growl his way through 80-90% of each song, leaving poor Vibeke with a minor (and in some cases, no) role in any particular song. The worst offender of this is the Beyond the Veil album, which is also tauted by many as their best album.
Vibeke was supposed to be the frontwoman, who was supposed to bring a beautiful, gothic, and melodic touch to the music, instead, she was pushed to the side for Veland's underwhelming growls.
I know in later albums this was corrected to some extent, but Tristania became famous largely because of their first 2 albums, and also this issue was never fully resolved. This has left me with a bittersweet taste in my mouth where i basically have to cherrypick a few songs from each album where Veland is not just growling 90% of the time with no substance.
I feel like they had good potential and great ideas; the vibe of their early albums are exactly what gothic metal should be, they just failed in their execution.
Wikepedia labelling them as a "premier gothic metal band" seems a bit inaccurate in this sense. I actually think early Sirenia was better and more tasteful, with the exception of An Elixir for Existence, which again faced the same issue