I think the OTs FX get a bit of a bad rap to a certain degree, the filter is excellent and easily on par with most other digital filters and some analog ones too, it is pretty flexible and sounds VERY nice IMHO.
The dark reverb is pretty good too, yeah it ain’t a Strymon but it does the job of adding all manner of ambience when you dial in the parameters correctly.
The spring reverb is quite trashy by nature and good on snares etc.
The chorus, spatializer and phaser are serviceable, not going to compete with dedicated units but add sufficient movement/modulation to pads etc.
The lo-fi can be good for adding grit or weirdo sideband harmonics, or completely destroying nice sounds.
The comb filter is really nice with convenient semitone tuning to turn mundane sounds into something a bit more interesting, good for FM style wavetable shenanigans and plenty more besides.
The delay is pretty flexible, key is to set up the filters accordingly.
The eqs and compressor are very handy for shaping tonal or transients respectively, for drums in particular I find them perfectly usable.
In short the OT fx require a bit of experimentation to get the best from them, it isn’t realistic to expect them to be world class standalone fx, but they definitely are useful and in most cases get the job done just fine.
The later fx in the DT, DN, AR, A4 are probably a little more refined, but don’t rule out the OT as it can sound excellent in its own right.