1. "Scaling", or rather the fact that Crit Chance does not affect all abilities (ie: Lava Burst). This means that when there is a high crit chance from gear or buffs the value of casting Lava Burst vs any other spell is lowered. Ascendance has this added as a damage bonus based on the crit %, which is a band aid to make the talent not completely horrible.

1a. Overload, the mastery effect, is still a % chance for a fixed damage event so it is possible to cap this at 100%. This has happened a few times, resulting in tweaks to overload to reduce the chance % from mastery in exchange for increasing the damage. However each time this happens it reduces part of the "spec fantasy", which was best shown when Warlords introduced Multistrike & mastery changed to a different effect. The number of additional overload animations triggered from multistrikes were vastly decreased.

2. A "decision conflict" within the rotation where deciding to cast Earth Shock, which is a resource spender that uses all of your current maelstrom, vs casting Lava Burst when it is off cooldown but also generates maelstrom, comes down to a maelstrom value tested in sims by trial in error. I don't think any other class/spec has a combination of a complete resource dump combined with a cooldown spell that also generates that resource. It's also affected by mastery, since overloads also generate maelstrom.

3. There's no way to "pool" resources for any period of time, so you can't save mana for a burn phase like Arcane can, or even save a soul shard or two like Warlocks can. The cooldown of Lava Burst combined with Lava Surge resetting that cooldown means that you have to use it ASAP or risk wasting damage potential. With Earth Shock you want to use it as little as possible while avoiding capping maelstrom, so you can't save it for movement or AoE (via Earthquake which also uses maelstrom) that happens in a few seconds.

4. The potential for certain talent combinations to result in an inverse of Ascendance: spamming Lightning Bolt (aka the filler spell). I can't recall whether this is still a problem, but it was an issue with Storm Elemental in the past (which gives a cast time reduction buff to Lightning Bolt when active).

That's most of it off the top of my head, and some of it may not be correct as I haven't played WoW in 3 years, and re-rolled from Elemental at the end of Warlords.