Heroes have slightly, with emphasis on slightly, more DPM than demon slayer when you fund them due to their scaling with final damage. Demon slayer have more DPM than heroes unfunded due to higher base DPM output. As an overall bosser, even when funded, Demon slayer outclasses heroes. Heroes really only excel at DPS and being decently tanky (and their dps funded barely eclipses DS). Demon slayer not only have amazing DPS, but they have amazing dodging/mobility(chaos lock, glide, dash), amazing sustain (50% hp Every 10 seconds), binds, waveclear with demoncry, iframe with metamorphasis, and the ability to ignore weapon cancel/dr with metapmorphasis. Good bossing classes have great mobility, decent dps, great utility, good sustain, and good tankiness. Heroes really only excel on two of those traits whereas Demon slayer excels on every single trait listed making demon slayer a much better overall bossing class. In terms of mobbing demon slayer also beat heroes easily. Heroes mobbing is very bad since they cant cover much of the map. DS can use demon cry or infernal concussion+chaos lock to level down the map within seconds.
Heroes don't have much defensive skills either, idk if they're squishy, they're just not as tanky as their explorer warrior brethren. DS has an easier time bossing, idk if that matters to you. Matters to me because I'd rather have lag insurance than "git gud".