I was actually very pleased with how the new forge turned out, from the moment I started playing it. The terrain pieces, the scripting, and all of the new tools and additional pieces make this forge vastly different from any other.
I don't think that the scripting is buggy at all. When first experimenting with it, I had expected the system to be inconsistent like some of the "advanced" settings could be in the previous games. However, once you understand the system, everything works fluidly and logically, every time. There may be some delay if you're building a more complex system that, for example, triggers several different objects across multiple channels. But it will always work- the system was implemented just fine, but for this one issue.
I too have noticed some frame rate drops and performance dips in forge mode, but I actually think that this is mitigated in custom games. I have forged a tremendous outdoor map, using huge amounts of the terrain pieces and long sight lines (so many objects are being rendered at any given time). While there is some lag and red on the performance bar when looking down from some areas in forge, there are actually no from rate drops when playing on it in a custom game.
The object count would be just fine (in my opinion) if certain objects, including spawn and objective objects, switches, and timers, did not count towards your 650. But they do, and thus you're right that the object count is quite restrictive.
Keep in mind that Halo is still focused around relatively small sized maps, so you shouldn't really expect to be building anything on a Battlefield-like scale in the game.
To summarize, I'm not sure what exactly you were expecting, but the new forge does seem categorically improved to me.