Ok, since Flutter Mane didnt work out so well, lets take a STAB on Chien Pao.
Personally, if you think Flutter Mane is fine in mono, it follows that Chien Pao is also alright in Mono, due to the following reasons:
1.) Chien Pao only has 4 Moveslots, less if you run Swords Dance
Without Sacred Sword, Kingambit takes on Chien Pao quite readily. Without Psychic Fangs, Toxapex sits on it. Without Swords Dance, Physically defensive walls such as Corviknight, Ting-Lu deal with it [Corv can even use this momentum to setup Iron Defense to brick Dark teams if needed]. Without a Choice Band, Quagsire sits on it and can punish incoming switch-ins with Toxic or litter the floor with Rocks. Without Sucker Punch, Scizor deals with a SD'ed Chien Pao, and without Ice Shard, Breloom, Choice Scarf Flamigo, and Choice Scarf Great Tusk eat it up. Because of the above statement, Chien Pao will always be walled or countered by something, similarly to Flutter Mane.
2.) Choiced Sets can be taken advantage of and are easier to wear down
Choice Band is certainly destructive, being able to 2HKO Pokemon like Corviknight, Ting-Lu, basically everyone I mentioned. However, its now weak to Rocks and cant counter the aforementioned Choice Scarf users I mentioned earlier which Dark teams hate facing. So, the response to a Choice Band Chien Pao would be to sack something and get in your scarfer or bulky Pokemon who isnt KO'ed by it, much like how you do it with Fluttermane. Its also worth noting that Dark has no access to entry hazard removal at all, not even Magic Bounce.
Wait, isn't this familiar? Flutter Mane also 2HKO'ed its defensive answers if it had the correct move. In playing, you only have one team against the flutter mane's team, and you don't care if Fluttermane is walled by something like Dondozo if its currently smacking your Ground team with Psyshock/MystFire.
Oh, and Chien Pao isnt even strong enough to KO fluttermane, so yeah, think about how CB Weavile did the thingy last gen, its pmuch the same:
252 Atk Choice Band Sword of Ruin Chien-Pao Ice Shard vs. 0 HP / 0 Def Flutter Mane: 201-237 (80 - 94.4%) -- guaranteed 2HKO
3.) A lot of revenge-killers arent weak to Chien-Pao's priority moves + There are indeed a lot of scarfers/priority/bulky mons right now.
(First Impression beats Dark anyway)
(takes one in a pinch)
(If it has no Sacred Sword)
Electric: (Personally don't use it, but
is so thick so :D)
(That now sometimes uses scarf)
Fighting: (literally almost everyone)
(This is also a bad matchup for dark)
(If it locks in to Ice Shard to kill fluttermane)
(If no sucker punch)
(But Ground had a problem with Weavile last gen, too)
(but like, Normal isnt good)
(They have a weavile problem last gen too)
(if not banded) /
(if no sucker punch)
(now running Jolly for Fluttermane anyway)
To reiterate what was said in the flutter mane discussion, you dont need a defensive switchin for everything, so Chien Pao seems to be fine. Thank you for reading