I highly recommend the Sportcrafters Overdrive Pro Rollers.
Purely mechanical, and by far the best bang for buck in rollers out there.
CNC machined aluminum drums.
They come standard with "Cadence" drums.
The "Overdrive Pro" version comes with a "resistance" drum.
You can also buy an "Inertia" drum which allows for coasting.
I purchased the Overdrive Pro version and the Inertia drum and that gives you a lot of options.
For pure cadence low Z2/Z3 work I simply swap out the "Resistance" drum for a "Cadence" drum and spin at the desired steady effort.
For a more intense workout you can swap in the "Resistance" drum, with the red end facing to the left, and it offers a moderate resistance where you can work yourself easily up to sustained threshold efforts and possibly beyond depending upon your strength.
For actual anaerobic interval training you flip the "resistance" drum so that the red end is on the right.
The faster you go, the resistance increases exponentially, they're rated to handle upwards of 1000 watts.
(Magnets within the drum extend outwards at higher RPM's inducing drag you need to overcome)
You can absolutely accomplish interval training on these rollers. It takes less than 5 minutes to swap out a drum, and their customer service is superb. I had an issue with one of my drums endcaps, I sent them a photo, and they shipped FedEx a new drum the same business day.
Made in USA.
I bought mine on sale with an "Inertia" drum for a total of $425.
They also fold up neatly out of the way.