For deep carpet cleaning, I guess my niche vacuum is a lightweight Oreck clone, like the now discontinued Hoover Platinum Bagged lightweight upright. Simple but effective low amp motor in a fan first configuration with a large up front disposable dust bag. Anything like the old classic Eureka Ultra, Eureka Boss and Hoover Elite. With the handle doubling as a dirt path to the bag. No on-board tools to complicate the cleaner and make it heavy.
For bare floor cleaning, my “niche” cleaner is anything that resembles a Swiffer Vac and Dust: any vacuum that features an unobstructed front suction nozzle paired with a disposable or washable mopping pad. That’s my niche bare floor cleaner.