252 SpA Life Orb Sheer Force Nidoking Flamethrower vs. 252 HP / 252+ SpD Bronzong: 161-192 (47.6 - 56.8%) -- 37.5% chance to 2HKO after Leftovers recovery
252+ SpA Life Orb Sheer Force Nidoqueen Fire Blast vs. 252 HP / 252+ SpD Bronzong: 203-239 (60 - 70.7%) -- guaranteed 2HKO after Leftovers recovery
Arguably, you can make a case regarding how it's much less common than moves such as Sludge Wave, Ice Beam, Earth Power, or Thunderbolt, it should be noted that from the moveset stats for Nidoking at 1000, 1500, 1630, and 1760, the percent usage of Flamethrower on Nidoking is 26.289%, 26.039%, 23.300%, and 17.219%, respectively. Similarly, you can make a case regarding ignoring 1760 metrics simply because the playstyle falls more in line with Tournament play (everyone running weird shit to one-up the opponent). Regardless, considering that 25% of all Nidokings used in UU potentially run Flamethrower, Bronzong's credibility as an answer to Nidoking comes into question.
Regarding Nidoqueen, her metrics are much different since she's more of a Offensive Support Pokemon. She's very cookie cutter and this becomes more apparent the higher you climb up the ladder. Bronzong is in fact a good counter to Nidoqueen, but just being able to counter one Pokemon doesn't really justify Bronzong's niche. Although your point regarding Bronzong's ability to switch into Grass-types very well is valid, specifically Shaymin, it can't really scare any of them out in any way except for the Gyro Ball of course.
Your final claim is honestly an odd point to bring up. Although the stats indicate Bronzong as a very bulky thing, he does lack recovery, which significantly cuts down its ability to deal with offensive Pokemon. Analyzing the Viability rankings as well as usage stats, Bronzong suffers against a lot of the top tier threats in UU right now, namely:
- Hydreigon
- Anything with Knock Off (some Fighting types)
- Chandelure
- Blastoise-Mega
- Krookodile
- Suicune
- Absol-Mega
- Haxorus
- Infernape
- Nidoking
- Sableye
And this is just the top of the viability rankings list. Essentially, every single one of these Pokemon has a way to significantly damage or cripple Bronzong.
Honestly, if there is one benefit regarding Bronzong, it's that it is almost a staple on Trick Room teams due to Trick Room, bulk, and Explosion, grabbing the team significant offensive momentum. However, Trick Room teams are technically considered niche, and to be a niche of a niche is kind of an odd way of justify placement on this list.