What if

I feel like it's not worth it because if there WAS a good christmas banner, then I can't summon on new year banners

With the IVs I want too? If yes, then I'd do it instantly.

Whichever units I want? I'd do it.

I have 100 orbs so that's 25 perfect rolls. Looking at FEH Keeper, I've got 24 focus units that I've been aiming for in 2018. So, basically a whole year's worth of good pulls? Too good to pass up.

Not in a life time, i'm happy with my main account

I dont see a limit so give me 10 Kliffs and thats a huge yes

The character guaranteed to me would have to be exceptionally good and also extremely hard to get (like, several times worse than a unit on a 4-unit focus) for me to justify the risk. I would need the guaranteed character to beat any potential meta shifts better than the next three units for a year.

Plus we need to consider that I could just roll without guarantee for this unit that I really want. If I can expect to fire 200-250 orbs to get this unit (new red legendary, for example), I can still get the units who are to be released in a week...in 6-7 months at the latest on a legendary banner, but often sooner on skills banners or VG banners or other random banners like a surprise hero fest. So the difference between spending 250 orbs to get my desired unit and being guaranteed the unit needs to outweigh anything any of the three next additions to the game could do for my barracks for at least 5 months, but a more reasonable number is 8-9 months.

So if we take a unit like Surtr and make him the only red on a banner with four focus units, I could consider it. (Theoretically, anyways. Reinhardt and Ophelia and swordbreaker mages would probably wreck him if he was red.)