Tailoring content to order in an MMO would exponentially multiple the necessary labor.
Not sure how much programming you've done, but it's a lot easier than you would think. To begin with, no one is advocating customizing it at the individual level, but even that wouldn't be that difficult, since it is just a matter of adjusting variables. All mechanics, triggers, etc could be identical, with changes to timers, amount damage, etc. It would only require a 2 tier, maybe 3 varset to make distinct difficulty settings. This is already done with solo MSQ duties. Creating it initially and doing the coding would be easy. Getting the tuning able to be universally applied to any instance wouln't be too bad, the only labor intensive component would be manual QA testing of all instances.

I would posit that the worst part would be creating the design documents.

There's nothing "exponential" about it. To personalize it, creating a performance tracker, which serves as a variable multiplier is all it would take.

None of these are technically challenging.