I want to note first, that I have no thought to contradict the current edition of 'Stoic' ideas that are giving people comfort these days. Maxims cherry-picked from Marcus Aurelius Meditations are mostly perfectly good ideas, if taken out of context. My interest is in understanding my opinion of Stoicism as a technical philosophy and why I have, on that level, a generally bad opinion of it.
Also worth note is that Marcus Aurelius was the last in a series of four or five important Stoics. Zeno and Epictetus are the two others who influence my thinking most. (Though Seneca, another important Stoic is interesting in that he was tutor to Nero.) They varied significantly in their interpretations of Stoicism.
I am correct in my recollection of the fundamental Stoic reliance on God as a motivating concept. Though the Stoics were (like me) materialists believing the opposite of Plato's notion that the real reality is the ideas that occur in you mind, somewhat inspired by the information of your senses. The materialists consider the world to be physical and our perceptions to be indicative of what is really happening. (Eg, the mishapen circle we draw is more real than the ideal, mathematical one we imagine and use for our geometry).
For Stoics, God is the soul of the physical world with most of the attributes of the rest of the Gods. For them, virtue is obeying the laws of that God/nature, whatever those are. Zeno and Epictetus took this to the extreme of considering anything that distracted from behaving virtuously to be anti-virtuous and thereby actually seeking discomfort. So, for them, and this is where I start to be annoyed, comfort and pleasure are to be avoided and love for others is no virtue.
It has the famous idea, inspiration to Aurelius and the modern maxims, that the good life of virtue is entirely internal to the person, ie, one's thoughts. The quip has been made that a Stoic could be happy on the rack, under torture, if he retains his virtue. They hold that Socrates cheerfully choosing death rather than recant his philosophy is the ultimate hero. A good Stoic is not troubled by the death of, say one of his children since that external event is no obstacle to this sort of virtue.
The way that the Stoics got to the idea that the only virtue is (put over simply) good thinking is that they believed in determinism. Fundamentally (which I mean literally to distinguish from the nicer ideas in pop culture today), the Stoics thought that God created the world for an inscrutable reason and that our actions were merely motions of the cogs. It is true that they also considered humans to be God's purpose but that didn't make our will any more free.
Every doctrine has stupid crap like that and I am happy to overlook it. My real beef with the Stoics is the passivity it suggests. It turns out that Aurelius was king in a bleak time (war, pestilence, earthquake, insurrection). Much of the point of the Meditations (written as a diary, not for publication, and which I have not read) seems to be the account of his ways of enduring life in a difficult time, ie, obeying the will of God (in a deterministic universe where free will is not a thing).
As Bertrand Russell characterizes it, "When the Stoic philosopher is thinking of himself, he holds that happiness and all other worldy so-called goods are worthless; he even says that to desire happiness is contrary to nature, meaning that it involves lack of resignation to the will of God."
Stoicism, then, provides a way to live with adversity and offers no guidance about how to avoid it or to make it better. This is appealing, of course, when times are difficult. I like the idea of controlling one's internal state to feel more comfortable but, when choosing a philosophy, I prefer one that offers ideas for a life that is better.