Opeth really doesn't use too many tritones. Actually, the rhythm guitars in pretty much any Opeth song are an interesting study in harmony and counterpoint rather than a "Chug-chug" that has been entirely overdone in metal ever since Judas Priest was the cutting-edge.
Vocals are completely subjective. I'll admit, it took me a while to get used to the growling parts, but that said I enjoy them well enough to listen to them.
As far as the guitar leads, very few of them are purely pentatonic--for the most part, they are diatonic. What they may lack in pure technicality, they make up for with melody and phrasing. Actually, they are very Michael Schenker-ish.
Anybody who rates a band soley on the ability of the guitarists to sweep-pick 64th notes is a moron anyway. If speed and technicality are your main concerns, watch Formula 1 or become a court stenographer and leave music to musicians.