Vampire characters are too powerful. If a VC player makes a Master Necromancer the general, he has to use sophisticated tactics, but with a vampire lord, you can win a lot of battles by just forcing your blender into combat with the enemy's center lines. Games Workshop is pretty bad at army creep. Tomb Kings was the second army book they ever released. Vampire Counts was one of the last books released. That means they are more powerful. The TK have a lot of cool ideas that rock in theory and fail in implementation.
Terry Pratchet book series. The entire world is held up by 4 elephants on the top of a gigantic turtle that is swimming through the cosmos (or something like that, I didn't actually read the series)
Vampire Counts are well balanced in 8th edition. They find themselves somewhere in the middle of the pack. Unfortunately it is the TK that find themselves under-powered compared to the other armies. As for Vampire characters being too powerful, other armies can field a multitude of extremely powerful characters as well. For example (but not limited to): WoC - Unkillable Hortennse Lord or the Daemon Prince Bretonnia - Flying HKB lord Lizardmen - Oldblood or Scar-vet Empire - Van Horstmann's Speculum wielding character Ogre Kingdoms - Fully kitted out Ogre Tyrant DoC - Greater Daemons Dwarfs - fully kitted out Dwarf lord can be terrifying (although admittedly quite slow) Dark Elves - Reverse Ward save character And that is before we even consider uber powerful spell casters, of which there are many. So yes, a Vampire Lord is extremely powerful, but fluff-wise it should be and game-wise they pay a lot of points for it. They are definitely worth it, but the VC army is not overpowered because of them.