I am a fan of modern flat design and despise non responsive fugly outdated website. But i found most (99%) offers are super outdated.
My first question would be, how well do they really convert with the younger generation (or do you not bother to advertise to this age group)?
And my 2nd question would be, do you design a landing page to match the site? Of course I should be testing this myself, but your input would be helpful.
Something like: Does modern LP > outdated site = LP CTR (high) > Site CR (low) ?
Generally, matching LP design to offer page design does work well - but better yet, matching LP design to that of the site your traffic comes from.
Modern designs vs ugly, simple landers is just a typical split test.
Never underestimate the ugliest of the ugly, sometimes they just work...
For most activities, the landing page serves as a bridge between your initial and and the advertiser's website. Generally speaking, the more seamless you can make this transition for the user, the better.