Heatran is a pain.
You don't want to start with it.
You can't retreat it, so you end up losing a ton of deck space to Warp/Switch/flippy Level Max (none of which you can play if Trainer Locked)
If you don't Bench Shield it, it will die to Flygon.
If you DO Bench Shield it, you can't Unown G it, so Heatran will die to Gengar, or be switched active with Luring Flame or Inviting Trap (yep, Flygon decks own Heatran either way).
Gengar X's Level Down makes Heatran cry.
Is that enough reasons to avoid it, yet?