To me Showtime is a marker.
It seems like all the great stuff we expected from Season 7 degenerates into Chaos after Showtime.
All the dark mysteries that demanded resolution up to that point are forgotten (what was the significance of the symbols on Spikes chest, why did what's-her-name the slayer run out of the house, what happened to the Home Improvement kid who got bit by the demon, etc.), played for laughs (ha ha we were just joking about Giles not touching anything), or anti-climaxed (Wood's just a dead slayer's son. Ho Hum).
After Showtime The First Evil is no longer a mysterious, dark and terrible force. It's played more for comedy relief. The episodes themselves veer off the arc willy-nilly in any direction that seems convenient at the time (I like Amanda, but did we really need her?). Also, more and more things in S-7 start making less and less sense.
Up until Showtime Season 7 had the potential to be something classic. After it S-7 it's only interesting. Just my opinion of course.
They don't seem to feel any obligation to explain stuff that doesn't make sense after Showtime, so you pretty much have to make up your own explanations. For example I'll offer you my extrapolation of why Ubie is more powerful than the ubervamps of Chosen.
When Spike is bled over the seal, it is done in a ritualistic manner. Symbols are carved into his flesh. A special ceremonial knife is used. I'm going to allow myself to assume this ritual gave Ubie added power.
Here's an interesting behind the scenes factoid concerning Showtime. When the episode first came out many people noticed it did not appear to be SMG's voice in the mindspeak scene. Other's swore it was. I swore it was Michelle Trachtenberg's
It turned out people who received the broadcast early did not get the same broadcast the United States received. (Sorry. I can't give you a link for that little gem. Can you just believe me, when I tell you, I had one, but I lost it. I remember thinking it sounded reliable at the time)