I genuinely think that either policy choice (PC led politics, NPC led politics) is fine, as long as appropriate controls are in place to ensure that the downsides of said policy choice are accounted for, and as long as the staff team remains consistent with the rule.
Stagnation/Turnover
These are the same issue, just one or the other side of the coin depending on which choice you make.
With a staff NPC in a leadership role, stagnation of the domain (whatever game set there is) becomes a serious potential issue. These characters also tend to be either too competent (leaving no room for a lot of things), or too incompetent without recourse for the characters (which becomes a suspension of disbelief problem).
With PCs in leadership roles, the turnover is generally intense. The pressure on the players of these characters is also intense, without active steps being taken to combat it.
Character Accessibility
When your leadership characters are played by staff, it limits the access people have to them (or the quality of the access is shit), because staff have eleventy billion other things they have to be doing. Ensuring access is HARD. Ensuring FAIR access is impossible.
When players play leadership roles, they typically only have that one character there (or maybe an alt or two). LOTS more access / involvement can happen. Including things like the seven hundred year old vampire prince of the city going to Starbucks and pretending to sip coffee while talking about Becky's poodle, because their priority is RP.
Plots
With staff controlling the leadership roles, these characters can be a source of plots / information for plots. They do not, generally, get to participate. When players control the leadership roles, you have the seven hundred year old vampire prince of the city helping track down a guy who robbed the convenience store and stole Becky's poodle.
Getting things done
When staff controls leadership roles, handling things that involve leadership roles can be a +request. When it's a player, player personalities and conflicts get involved. As well, you either get mini staff (that you didn't really choose), where they have responsibilities and requirements on them, or you don't, which gums up the entire works. PC leadership means that when IC laws/etc get broken, you're going to have players dealing with this.
Agency
Players in leadership roles give the orgs they are part of far, far more agency in determining the direction of their spheres of influence. Staff in leadership roles remove a significant amount of agency from the player characters that are part of their sphere of influence. Mind, this isn't wholly a terrible thing, as it allows for significant control of possible thematic drift, and so on.
Engaging Characters
PCs are like 99% of the time just going to be more interesting than NPCs are to regularly engage with. They're "more real" people because they get played a ton more, and have personal connections with characters that NPCs don't really have time to have. While NPCs get crowd-bombed when they step somewhere accessible, PCs are just part of the tapestry of PCs, so they can have rich levels of involvement.
All of these varying points have different solutions that are more or less work. This is not even remotely an exhaustive list, I'm just trying to illustrate the breadth of differences. Both are 100% workable, valid, reasonable ways to go, you just have to pick one and then address the logical consequences of that choice for the game in question, your staff team, and your players.