OK, so... "leveled with full pve subtlety + some assassination build. Never had any problems what-so-ever" - that's just pure crap and lies. If you're saying this, then you either leveled through dungeons, or with groups, or you leveled veeeeeeeeeeeeeeery slowly.
- Ambush chunks everything to half health - no, unless it crits, you're overgeared and you're killing low-level mobs. As Combat, you open up, use Slice and Dice and finish with a quick Eviscerate.
- Soloing elites with double Evasion - how? You have no damage. I remember when I tried Sub leveling years ago and soloing elites was more stressful to me than outdoor PvP. Combat just zergs elites like they're normal mobs. Sub PvE is not harder, it just doesn't exist. You have the extremely weak Hemorrhage, the extremely expensive Backstab, low poison damage, low auto-attack damage. You're basically relying on your Ambush critting - otherwise your kill speed is painfully slow.
- Exploding mob after mob - as soon as you engage 2 mobs or more, which happens very often in leveling, you have to use major cooldowns or you lose most of your HP or even die. Combat barely has to use cooldowns and can handle several times more mobs than Sub or Assa.