When talking about oncogenicity, they mean primarily embryonic stem cells (ESC). ESC - cells obtained from the intracellular mass of a blastocyst (in a human embryo it is 5-6 days of development). The ability to produce derivatives of all 3 germ layers and all ~ 210 types of cells, except for reproductive cells (pluripotency) is one of the criteria for ESCs. To prove that the cell culture you have obtained consists precisely of ESCs, it is also necessary that such cells, when transplanted to athymic mice, were given teratoma in about 90% of cases. To obtain ESCs, the whole embryo is transferred into culture, the zona pellucida is dissolved (this is the former shell of the egg; normally, after a while the embryo "hatches" from it) or damaged, for example, by laser incision, after which a separate cell is removed from the intracellular mass by micromanitspulation or a whole embryo. In the case of a whole embryo, the cells of the trophectoderm are removed by microsurgery or immunosurgery (the part of the embryo from which extraembryonic structures are formed, i.e. not included in the body of the embryo) and the resulting cells are cultured.
Why did I write this long paragraph about receiving, which seems to be irrelevant to the subject of the question? It is important to note that when receiving ESCs, all stages are controlled and during them there is no gross damage to the embryo. It is believed that from the abortive material (the SCs obtained in this way are referred to as fetal, not embryonic) we no longer receive true ESCs, but it is not clear what a mess of cells. One of our lecturers said that fetal SCs are no longer pluripotent.
As far as I understand, this is due to the fact that the SC niche is important: a specific microenvironment formed by cells and extracellular structures (extracellular matrix, specific intercellular contacts and a set of signaling molecules), outside of which the SC cannot perform its function. The niche of the SC is a very finely tuned structure; it influences the SC by many, sometimes very unobvious factors. As an example: in addition to the known signaling molecules, the composition of the matrix, the rigidity of the substrate, its geometric characteristic, which determines the contact area of the cell with the matrix, can have an effect. These are mechanical factors, the cell can recognize them through special protein complexes that attach the cell to the matrix - focal contacts. Focal contacts can, with their intracellular part, "attract" many enzymes (among which the key one is FAK, focal adhesion kinase) capable of directly (MAPK-kinase cascade, matrix-nucleus connection: focal contact - actin - Nesprin1 / 2 - Sun 1/2) or indirectly (small GTPases of the Rho family → SRF transcription factor) to influence gene expression, etc. maintain stem quality. Data are accumulating on the influence of even the electrical characteristics of the medium on differentiation. Obviously, during abortions, this whole delicate system of maintaining stem ability can be disrupted, and when extracting, partially already committed cells are obtained, and onwhich way exactly - you cannot know until they are already terminally differentiated.
Stem cells can become cancerous, because they can also divide and are undifferentiated. Once such a stem cell can begin to divide uncontrollably, rebuild metabolism to an anaerobic type of nutrition, and that's it - the cancer cell is ready. Although cancer cells are constantly occurring in any body, the immune system fights them off. However, how the immune system will react to the additional emerging supernormal cancer cells is a question. Maybe it won't.
Any foreign cells are immunogenic, they will be rejected, so they are not very convenient to use. In principle, it is now possible to make induced pluripotent cells from your cells to avoid this problem.
Man never learned to use them to the fullest. We still do not "know how" to direct these cells in the right direction, replacing our damaged cells. Many experiments, when SC were "sent" to the brain of a person with neurodegenerative diseases, led to oncology.
No longer relevant. "Just a couple of days ago" the Japanese scientist Shinya Yamanaka learned how to obtain SC from any cells. The mature cell is forced to direct its development in the opposite direction, to the stem cell. Embryos are no longer needed. Embryonic stem cells are already in the distant past, not to mention that it is no longer fashionable among scientists.
Well, if we talk about the use of stem cells in general, then the main problem is precisely the lack of study. Where they are well understood, they work well