Okay I'll bite.
It depends on the "ultra". Racing a flat/flat 50km is a lot like a road marathon and correlates very well to marathon "speed" (heck even a flat 50-miler or 100km /100miler does as well).
It's the mountains/hilly/technical trails that is not a specific neuromuscular pattern that slows stride rate, zaps power and efficiency (read specific running economy for flat roads). You extend that over 100km with lots of vertical change and add in trail technicality/altitude/alpine weather and you have a very different type of running...often mixed with "powerhiking."
I initially ventured into ultras because I finished mid-pack in the 2012 Olympic Trials with a 2:18. I know where i stand on the national level on the roads and on the track (even worse...I only ran 29:47 for 10km in college remember). I also thought doing some 25-30 mile long runs might help my marathon "strength." I think on the roads/flat trails that could be very good for a marathoner (although 22 mile long runs are usually sufficient enough with consistent high mileage (i.e. for me 115-120mpw is a sweet spot) and balanced Tempo Run workouts/speed workouts).
So in short, after my disaster at UTMB (to be fair I also ran like crap at the Speedgoat 50km earlier in the summer...over 18-min slower than i have in the past on the same course) I've really had trouble getting the leg turnover and marathon OT pace (5:18/mile) back. It's taken over 12 weeks of dedicated training now and finally (after actually touching foot on a track for the first time in years) I'm starting to feel like maybe I can run a sub 2:19 marathon again. It's like the few fast twitch muscle fibers I had went dormant and i actually lost stride length/power. I guess we'll see soon enough in Houston (fingers crossed with the weather as I think it's going to be close!).
But right after UTMB and doing some other long 100km+ hilly/mountain ultras I've for sure felt slower and uncoordinated in those first few months. Turns out slogging around in the mountains at 12-min mile pace isn't very specific to running low 5-min pace on a flat road. It's all about specificity in training, periodization and training focus. Variable Running Economy.
You don't see guys that smash 100-mile/100km mountain ultras come back down and race super fast road marathons that often. Max King has probably done the best of us moving down from 50-milers mainly, but he always seems to have a 2:14 type of marathon in his wheelhouse! "Moving back up" from road marathons up to 50km/50-mile "runnable/fast" trail ultras is an easier transition to make actually. The hills matter and the musculature of your legs matter as well as your running form/power/stride (and nutrition).
I can only speak from personal experience (mainly my failures like missing the OT standard in 2016 by 12 seconds and having meltdowns in 100-milers), but I love all types of distance running events and it had been fun to mix it up (despite sometimes not being in my best interests in terms of earning prize money).