The question might be how long these materials might be in use. If it is for a short term, then it probably makes not difference. But for the longer term, you could have trouble. Upgraded devices, new devices may require newer standards. But this has moved pretty slowly so far.
That said, it is much easier to maintain documents that use CSS than all these inline tags. One change in CSS is much easier to do than searching and replacing hundreds of times and potentially blowing up the formatting because of something you did not remember or anticipate.
It may hinge more on the devices than on standards, since none completely adhere to standards, and some are worse than others.
Be sure you are previewing all your fancy formatting on the target device(s). A lot of fancy formatting is lost on small devices of any sort.