After all the hype has died down a short time from now, I think people will see the series for its true value: a meticulously crafted piece of entertainment, authentic and mesmerizing, but ultimately limited by its format and its source material. Others have said plenty about the good side of this series, and they're all true, so here I'd rather focus on the shortcomings.
First, the format. I don't know the economics behind it, but all the series made nowadays are either in 8-episode-per-season form, or as 10-episode miniseries. Why? With a sprawling set of characters and factions, with so much interactions and potential for interactions among them, how can a 10-episode miniseries portray it in a satisfying way? Why not do the thing that Asian television channels have always been doing, which is to make 30- or 40-ep long sagas? It is really limiting in terms of what can be portrayed and what must be left out.
Second, the novel itself. The biggest issue here is the lack of logical rules which govern the world building. Rules are either unclear or not well established, leading to a fictional world which feel like it could go anywhere by the arbitrary whims of the author. Episode 9 brought a glaring example. After publicly allowing Lady Mariko to depart for Edo with Toranaga's family to preserve his "honor" as a host, Lord Ishido at that very night sends assassins to kill her. There's nothing to be gained from it, since the whole point of not letting her go is to avoid the dilemma of dishonoring himself as a host or risking the great families' revolt. Now by killing Lady Mariko in this blatant and very-much-dishonorable way, Lord Ishido is getting both scenarios.
So many other questions abounds: Why is Toranaga, being the president of the Regent Council, so powerless and clueless as to allow himself to be politically maneuvered into a corner by mere bureaucrats and lesser lords? Why are there great Lords with essentially no military or economic power to back them up? What roles do they play other than as talking heads? What is at first promised to be a multi-faction political struggle turns out to be a two-faction back-and-forth. What a letdown. Why does Toranaga bring his army to a small town of Anjin, which is then conveniently done away with by an earthquake (by the author)? Why is Ishido's political usurpation, so important to the plot, only portrayed by mere implication and not fleshed out, so that it is more believable? Is there really no other way that Toranaga could act without sacrificing his best friend and his son? It is never laid bare. Why is there no outside backlash to Sugiyama's assassination? What's the point of the character of Omi?
So in short, a few problems seem hard to ignore:
_Characters act in an illogical way with no good explanation
_Things that should be fleshed out isn't.
_The power dynamics is not made clear and the complexity is increasingly reduced towards the ends.