I can attest to the negative reviews. Having worked deeply with their implementation team I got a good sense of their organizational structure and processes around prioritization, and client tactics. etc. I wouldn’t go as far to say the company deals in fraudulent work. However, the product managers, designers, and developers - of which are mostly outsourced to resources that barely know English - are junior at best. Considering their hourly rate you'd think they’d assign a bit more expertise and seniority to their accounts.
Communication with their team was largely painful for their clients mostly because their development resources are remote in other countries. Seldom did their developers deliver anything on time and within scope, and most project status updates consisted of what didn’t get completed with a longwinded explanation of why they’ll need more time (and a lot more money) to get something done - which makes sense because this is predominantly how they stay in business.
Designers like to tout they have industry experience in whatever industry their clients are in. If this isn't a flat out lie to obtain clientele I don't know what is. If you're a designer looking for work I'd say this is a good place to start because you'll be using the latest design tools, and also get a good sense of the latest tech out there to prime your career. But I'd try to keep your time there short.
If you’re looking to start your career in Product Management I’d stay away from Sidebench. Their product managers are glorified Project Managers - make no mistake, there’s a huge difference - and their execution team depends heavily on client product managers to do the leg work for them. They are neither data driven, nor do they possess the resources in house to make educated recommendations to their clients in the absence of data. Their prioritization methodologies are lackluster, elementary, and are pulled out of a handbook. Anyone can read a book on product management, but it takes PM expertise to apply them to their respective business environments.
I would also be hesitant to start your career as a developer or here. Like I said, their work is elementary at best, and you can’t expect to learn much if you’re just starting and your peers are in the same level as you. There is hardly any room to learn since your work is constantly being scrutinized by the client, and you are working in a waterfall environment. Better to try your hand at a company that has an actual product to build rather than doing agency work.