If a well disciplined approach is taken to sortie preparation there are no problems. If very current on the types to be flown you may be able to jump straight into the seat. If not so current and/or low overall experience on type you have to sit down with the books and do some careful preparation. You do have to work at multi-typing if you are to do it safely. However, some individuals do find it difficult to maintain currency on more than a couple of types at a time and so pilots need to be aware of their own limitations.
What is often more insidious are subtle differences between individual airframes of a given type. I fly some types whereby some have an ASI in knots and some in mph; that can be a real challenge.