I wonder how much of a benefit that is to offsite. Internal park transportation gives you a slight advantage in terms of getting to the gates (esp those who can walk to MK/EP/DHS). By the time you drive, park, TTC to park, etc, I think the actual on-site guests will have gotten pretty well into the park
We've attempted rope drop and Early Entry quite a few times. I generally agree that getting to the parks in time for EE is challenging. The things that tend to cause us to be late are often things that are beyond our control.
Most recently, the thing that has tripped us up the most has been MDE/G+ glitches when we attempt to choose a BG or G+ attraction at 7am, or it gives us a return time that bites, so we spend several minutes refreshing for a better return time.
Somehow between 7am and 7:30/8am, onsite guests are supposed to: 1. book BG/G+, 2. travel to the park, 3. get through security/front gate back-ups, and maybe 4. eat breakfast (when onsite food courts don't open until 7am).
We've done this successfully, but more or less- it requires walking to the parks or driving our own car, in-room coffee, and eating in the car/skipping breakfast/having food ready ahead of time.
On my most recent visit, we managed to arrive at HS before the start of EE, but - yowsa! - getting through the front gate took over 45minutes! All the entry points were held to a slow trickle. It was frustrating. Time was ticking past, and none of the lines were moving.
I often stay offsite the first night or two, then move onsite. The morning I move onsite, I have EE access, so I'm pretty familiar with attempting EE from offsite.
Even with having to check out of the other hotel,
we have a higher success rate making EE when coming from offsite. One reason is that most offsite places start serving coffee/breakfast by 6:30am. The other big reason is that driving = very predictable timing: usually less than 15minutes.
In my experience, onsite transportation tends to be slower, except when we walk to a park, go GF/Poly to MK via monorail, and perhaps Skyliner. (I don't have a lot of experience taking Skyliner in the AM.)
Entering Epcot via the IG used to be a slight disadvantage vs. the front gate, as the front gate often opened earlier. These days the IG is much closer to Remy, but the IG can still be slow going.