Ultraluminous X-ray sources (ULXs) are the brightest off-nuclear X-ray binaries, with luminosities surpassing the classical Eddington limit for a typical 10 solar mass black hole. While many ULXs are persistent, a few display remarkable short-term flaring events that provide unique insights into their extreme accretion environments. This talk will discuss specific ULX systems that exhibit transient and persistent behavior and illustrate how their spectral and timing properties can help disentangle the nature of the central compact object and the underlying accretion physics. A striking spectral feature of ULXs is the characteristic cutoff around 10 keV, whose physical origin remains elusive despite extensive observational efforts. A novel model is proposed in which high-latitude synchrotron emission from magnetized neutron stars naturally accounts for this ubiquitous spectral cutoff. This scenario offers a unified explanation for the cutoff and implies that most ULXs could host neutron stars rather than black holes at their cores.