Vantaggi
The people actually producing goods (software/hardware/services) are generally good. Benefits and salary are decent and are comparable to other big technology companies. The company has good controls for legal and accounting purposes (Sarbanes Oxley, GDPR, export compliance, etc.) My experience is that IBM is conscientious about equal opportunity for all and their facilities are ADA compliant.
Svantaggi
Too many levels of management and those who support management. Depending upon division, role, manager, etc, your work-life balance may be all work and no life. The corporate intranet is a mess, the search engine for said intranet is all but useless. Management moved everyone from cubicles to shared open-space just in time for SARS-COV2, all done in the name of "collaboration" when the reality is its about packing more people into a smaller space to save money. I've averaged getting a new manager every 18 months, there is often little/no knowledge transfer between managers and you have to train each new manager about what your role is.