The Geist morality meter is definately pretty well PC proofed, though there are a few points on which you could still degenerate pretty easily. Destroying a ghost is something a geist is frequently tempted to do, and raising the dead is something that can justify a degeneration while being a really cool background element. Sure, you do have to bend your PCs backstory a bit to justify having done sins against synergy, but flaws basically put you in the same boat.
Oh yes, with a couple of exceptions, unless you are pure out-and-out evil, and trying to drive down your Synergy all the way, then you rarely need to worry about committing sins against it. The only sins that I could see reasonably happening are three: reviving a dead person, destroying a ghost, and destroying a Sin-Eater (or, destroying a Geist).
The first one I would only see happening in a 'Pushing Daisies' sort of manner - bringing back a dead person (temporary or no) to try and help solve a mystery. The latter two I would only see happening in the case of the entity being patently evil. I plan on working such a scenario into my Sin-Eater's backstory, though whether or not he loses Synergy because of it (if I were GMing the situation, I'd provide the player bonus dice to resist degen, both because of the situation, and possibly depending upon the player's Virtue) is up in the air.
Besides, I think that if I can come up with a suitable enough backstory, then I won't need the 10 xp from the point of Synergy. I'm hoping for 10-15 XP, at the most, I think.
One main consideration with Geists is that they're built on a different power scale from other supernaturals because they don't have any extremely broad powers. They're just plain nasty within their niche, and incapable of mustering powers outside of it. On the other hand, we've got a lot of mages, (or so I hear) who take the opposite extreme in character design. For balance reasons, I'd caution you against using xp to pick up additional keys, because that can give geists an unfair jump in power level.
Well, you are correct that in most places, the powers of Geists are mostly suited for their own niche. There are a handful of exceptions (the Rage and Shroud Manifestations, as well as certain categories of the others), but yeah, mostly Geists are Ghostbusters.
And for the most part, any XP I get from Senti will be used in one of three categories: Merits, Manifestations, or maybe Psyche, depending on whether or not I spend 3 Merit points at creation to boost it from 1 to 2. Probably will, though.
When you're designing the background of a Geist, remember to think of your Geist and what it's trying to do, what its goals are. Geists are led around by powers beyond themselves quite deeply, or at least on a far more personal level than other supernaturals.
Mmm. *thinks* I'm coming up with a character concept now for a Forgotten Sin-Eater...perhaps I can rack your brain for some inspiration?
(The way I think of nWOD supernaturals in general, by the way, is that Mages have the most flexible powers, but with a signifigant risk of failure and limitations in pure power, Changelings are halfway between mages and vampires in terms of power level, and can generate a lot of power really fast at the risk of being caught by the fae. Vampires have the most robust power set and find it very easy to fuel, but they pay through the nose in flexibility. Promethians and Geists are roughly equal in power, slightly below vampires, though Promethians have the edge of being able to apply permanent boosts to their abilities, while Geists can rapidly arm a vast arsenal of powers, as well as having a major edge within their theme. Werewolves are pretty much the gimpy ones, without house rules to improve them, though the pack instinct makes them reasonably scary foes if you don't have a good supply of silver. Anything about werewolves goes double for hunters, though letting werewolves use the rules of hunter is pretty awesome.)
I couldn't really speak too much about this, given that I only have Geist and Hunter. But, the fact that Geists have a large assortment of powers does resonate with me.