Most people don't know the difference. Most people don't know how to properly set up their signal for it.
Dual Mono is taking ONE mono signal and splitting it into TWO outputs.
Stereo is taking a TWO (stereo) signals and sending the signal to TWO outputs (2 amps).
Mono Summing is taking a stereo signal and mixing that signal down into mono, and sending to ONE output. (i.e., Guitar > Eventide H9 (Stereo Out) > Strymon TimeLine (Mono Out) > Amp). This method does NOT give you the perfect balance of left and right outputs mixed down to mono.
MOST stereo effects do NOT have a 50% left and 50% right balance. Typically, it’ll be 70% Left / 30% Right or 60% Left / 40% Right, because it gives the effect of a stereo signal in a more pronounced way. In some cases, stereo effects may have an inverted phase or polarity. Summing these pedals will cancel out the phase and the effect will be removed from the mono sum.
If you MUST mono sum, use an active (buffered) mono summing box. Otherwise, use ONLY the LEFT output of the last pedal in your chain. It will be the closest resemblance of your mono sound.