So I started working on a project for the Spirit Shaman a while back and found that it probably is what is a low Tier 2, high Tier 3. Druid spellcasting by itself is not exceptional but it isn't terrible and it has a decent plethora of prestige classes it can hop into and can pull a number of tricks dedicated spellcasting druids can but the lack of Wild Shape and an Animal Companion definitely take a chunk out of it.
The weird way of acquiring spells and how it can easily cycle through them or find some niche to occupy makes it highly versatile and it can do something like holding its own with spontaneous conversion feats to be a summoner with more summons per day than a druid, but that doesn't really compete with what the druid can do. However, it has some really, really novel passive benefits which are game changers when relevant but pretty narrow in interest otherwise.
The split casting attributes is liveable, it really just depends if you plan on using things that give saving throws or not and how you feel about some.secondary class abilities. Even then, still salvageable to play, downside being there are even fewer options for dump stats and no real easy way to make up for them outside of the usual magic user answer of "More spells!" If it were purely Wisdom based it would improve but I don't think it would budge anywhere on the tier list and I do not think anything about it would break even slightly.