Based on your statement - "we use the rooms only to sleep", then you are not comparing apples to apples. That is because there are no Universal apples to compare to the 4 park, 2 waterpark WDW vacation with free dining. They are not the same place, and you will not have a comparable experience. It sounds like you want a week-long, commando style vacation going from early entry to close in theme parks, eating while you are there, and sleeping in a room that is dark at night. That simply does not define a week long, onsite stay at Universal. If you want something similar (and it is still not), get a cheap, clean hotel on or near International Drive ( I would actually recommend a Sea World partner hotel), you can get a package with free dining one day each at Sea World and Aquatica included, Quick Queue at Sea World and other perks. Get the 5 park pass, Aquatica is just as good as a WDW waterpark (opinion only), Sea World is fun and adding a new coaster next year, and you can spend a couple days at Universal. We stayed at Faifield Inn and Suites at Sea World, and the included continental breakfast was more than sufficient, although we always enjoyed our breakfasts at Carribean Beach as well. There are plenty of budget minded food options on I Drive, and a day at WonderWorks is inexpensive and enjoyable.
I will let others chime in, but we have done the free dining Disney immersive vacation, plenty of onsite stays at Universal, and multiple Sea World/Discovery Cove centric vacations. They are all different and in the same ball park of vacation dollars. I know Discovery Cove will cost us more, but it is awesome and we have to wait and save for an extra six months to do it, just the way it is. But, we always seem to leave our Universal vacations wanting more. On site hotels and pools rock, dining is incredible, the parks are great and relevant and changing/upgrading each year, CityWalk is a blast, Blue Man is wonderful and always funny, and we don't have to plan our food 180 days in advance, rides 30 to 60 days in advance, and we can enjoy and relax at that same time.
And, most importantly, there is this value enhancer that you might hear a lot about - Express Pass!!!! No more commando for us. Quite frankly, if Disney had anything similar and I had to pay rack rate at deluxe Disney hotels, we would do it. Everyone has their opinion and this is mine - but if you took my family's favorite Disney rides from each of the four parks and put them in one or two parks, closely situated, along with the best Disney restaurants, and gave us unlimited fast passes, we would be all in. But, I am not wasting my dollars riding Stitch or Ellen's Energy Mover waiting 30 minutes in a certain portion of the park so I can make my ADR that I made half a year ago.
I stated in a previous thread that the only way I got my kids to WDW for the first time was the free dining fall package and we had a great time. My niece is a
Disney travel agent, so I get it. Those packages are a great value - and they might not be matched at other Orlando destinations. Hopefully for you and thousand of other families, Disney will continue to offer that package. But, there are other destinations and other experiences to try. My wife is the kind to continue to go to what she knows, I am a try a bunch of new stuff kind of guy. Polar opposites - yet for the past 17 years, it has worked.