I really wish you would have said that frozen sigil thing first so I could have stopped reading off the bat. Have you ever seen sigil used? you say that pub games don't matter but then you say he's built around overpowering his opponents via combo, yet less than 5% of pro teams are playing him in that manner. you are such a confusing person. You seem like the very last person in this thread to decide which types of games/heroes matter.
@puffler: Everything about the hero screams early game strength. all 4 of undying's skills remain annoying easily into midgame with his damage amp ult being useful all game long, as mentioned by someone above.
decay is great because you only need 1 point in it and it goes a long way. I don't think it falls off quickly because 4 str stolen per hero is a fuckton until quite later into the game. catching 2 people in it is almost like throwing an invisible ogre club into your inventory every 10 seconds, and that's just for catching 2 people.
tombstone can make teamfights impossible early game, but the new meta does seem like it can make it completely useless by midgame (leshrac, bristle, es, gyro, qop, sf are really common). In the pro scene, i'm guessing that playing undying is pretty much like saying, "Hey I'm gonna force the other team to possibly draft a lesh, so get a strong/mobile bkb carry to counter midgame". Then it's undyings job to use his strong early game to make space.