The nice thing with scenic bases is they're pretty.
The not-so-nice thing with them is they're ridiculously unrealistic. I'm pretty sure Calgar doesn't run around battlefields carrying a land raider over his head and I'm pretty sure Defilers don't carry wrecked Baneblade everywhere they go so they can perch on top of it now and then.
Where small scenic bases, like a space marine corpse on a killer kan base or something, clearly offer no advantages/disadvantages, I strongly believe that huge conversions shouldn't affect the game in any way either.
If it was me, I would say "Wow, that's an awesome model! But... for gaming purpose, let's assume it's just the regular one". If my opponent would then disagree and insist on getting cover saves and whatnots, I'd go find myself someone else to play with.
Decorative conversions are meant to be decorative and nothing more.
Now I'd like to add to the list of crazy things, but I haven't gamed enough yet to see anything over the top.