It is totally wrong to attribute potential actions and adverse effects of essential oils based on broad chemical classifications. Essential oils are highly complex mixtures of natural chemicals, and in fact many are so complex that they still can not be fully re-created by chemists. Many contain large amounts of unidentified chemicals and therefore the actions of such individual chemicals are unknown. As already stated, it is the trace chemicals which contain the most active fragrance and flavour molecules, and it is a fair assumption that many highly active therapeutic substances also only occur in trace amounts. Thujone does not exist as one chemical, it has isomers one of which is 4 times more toxic than the other.
Some oils contain a lot of one isomer and other oils a lot of the opposite isomer.
Therefore you must know precisely which isomer exists in the respective oil, and what the precise actions of the different isomers are. Even then you can not be certain of the effects of a potentially toxic isomer, due to the modifying effects caused by the numerous other chemicals occurring in the whole oil. Chemistry does have its uses when looking at essential oils, but it should always be secondary to the knowledge of the effects of the whole oil. Students are being told that the action of particular oils are due to x, y, or z chemicals. However most aromatherapy schools do not have a clue what the oil they are using actually consists of.