I dont know how helpful my advice will be on this topic given youre ground is likely substantially different than mine here in North Florida, but over the past year one of my major projects on my property is converting woods to yard. Its a major conversion, as the woods are thick in places, and have giant trees and stumps, tons of rotted and matted ground clutter. The last step is to plant grass seed in these areas. Im at the point now where I am planting the seed in many areas. Im about 20 days now into one large patch that germinated and broke through on day 7 after planted, which was excellent. Ive done all of this ground work largely with one main tool - Disc Harrow. This is of course after I have drug off all the leaves and matted crap on the top of the ground and scraped it down to bare dirt using a box blade. A freaking love my disc harrow. I was torn on getting a disc or getting a tiller, but went with a disc as it was safer to use it in the woods and in rough terrain with lots of roots and crap. If I take enough passes over something it essentially will have the same result as a tiller.
So my general process is to disc these areas Ive cleared several times, turn the ground to pure powder. I am working with sandy soil here though so this happens fast for me - and this is the difference for me. Just for reference on the soil Im working with - we got 11 inches of rain in 24 hours the other weekend from that sub tropical system that hit the south down here. Within 1-2 hours of the rain stopping all the puddles were gone here. Water leaches extremely fast, as its mostly all sand. The downside is little nutrients. But, for breaking ground its probably as easy here as anywhere. So, I disc it over, turn it to powder - to where walking over the area your feat just sink into the ground with each step. I then made a little drag using some lumber or an old fence post and drag it all level, then I plant my grass seed, then drag it over again just to cover the seeds with dirt.
How well would a disc harrow work on hard/clay? I dont really know. I know for my application here in florida a disc is a fantastic tool. I can pull it all in the woods and just break ground, mulch leaves into the dirt, its just a fantastic ground engaging churning tool. I also recently used it in a horse pasture to till me a 10,000 sf area for a new garden, and it broke up the thick bahia grass with ease. I just love how indestructible it is and how efficient it is. Drop it and go. Dont have to worry about damaging anything or hitting a bad root or something like I may with a tiller. It is perhaps a little slower than a tiller at prepping an area for planting but it gets the job done, and is more versatile to me than a tiller given I can drop it in all sorts of areas. Obviously Im recommending you look hard at a good, heavy disc harrow. Weight here is important though. The one Im pulling is actually rated as being too large for my L2501, but ive pulled it without any problem.