there's half a dozen of them depending on what's convenient to the story.It is not supposed to be "convenient to the story" but just uneven. She can be the silly girl when she's with Imoen and next moment she realises how much responsibility she has as a healer or by being involved with the PC, a person of great significance. She is still searching for *her* way and role in this - which is not easy with her background.
We are talking about the initial chapters in BG1 here - a period where the PC him/herself is not sure about the own role. She is supposed to grow as your character grows and as the main story starts to evolve. You will see her develop over the story the more she finds out about your and her involvement with the bigger scheme of things.
Sandrah makes the game easy..Some say it is the opposite, I heard it is far too tactical. My own experience is that it largely depends on some other mods you have installed and that make her gain more abilities (I found a mod giving her 4 extra stars for weapons and use of druid items...). Maybe we should compare notes if you post your Weidu.log from your current install.
I'm using the BWS tactical installation and I didn't do much changes to it. Just solved a few conflicts that BWS asked me to do and left everything else as they were.