I'm genuinely very grateful for my time at Silverpop, but I will not be holding back on listing the cons. Much of this I wish I had said during my exit interview but at the time I was too close to the situation, and this was my first real Software Engineering job. The time away and the job I currently have has helped me gain some perspective.
You can tell there used to be a really great culture with Silverpop. There are people working there that that have been around since the start, and they are some of the nicest people you will work with. You could blame IBM's "blue washing" process for crushing the culture, but I think it was dying off well before that. Silverpop just grew very quickly, and it seems like they made some big mistakes along the way. The teams are also very diverse, and you have no control over where you land. Some teams are abrasive and dogmatic to the point of being unreasonable, leaving you to feel like there's not much room for individuality.
There isn't any valuable cross-team communication, and it is often impossible to track down who made a decision and why it was made. Their UX team is a joke. The decisions they make break conventions. There was a point where their design was adamantly rejected by the majority of engineers, yet they were unyielding and we were forced to comply. They wield too much power with too little supervision.
Now for IBM: The Lenovo laptops they provide are garbage. They are huge and generally terrible to use. IBM's internal tech support and HR is utter garbage. Their intranet is huge, buggy, and impossible to navigate. You are forced to use IBM Notes (a.k.a. Lotus Notes, and no it's not dead). Normally you would think "it's just email" but IBM owns Notes, and the software is so terrible that it genuinely makes you lose faith in the company. Aside from that, their policies are global and since IBM is HUGE and international, you often find your self meeting requirements and doing training that is not even remotely relevant to your job.
IBM's career paths are outdated. They lump their employees into grades, and the documentation on your grade and career path is complex and confusing. It definitely felt like the only way to move up was to brown nose and play politics. They make you waste a lot of time with seemingly useless paperwork and training that is supposed to help you grow in your career. To be clear, I am a huge supporter of employees that encourage learning, but the mandatory learning that IBM pushes makes very little sense as a Software Engineer.
The biggest thing is the stress. Aside from all of the stressors already mentioned, during the acquisition it seems like Silverpop promised IBM some very unrealistic dates for delivering certain features. It definitely felt like the engineers were never consulted. There was a TON of pressure to deliver features and it seemed like every team felt like they were drowning. On top of that there were a ridiculous amount of meetings that were rarely relevant, focused, or productive.