What types of problems? Interoperability issues, networking conflicts, storage application layer incompatibilities, software maintenance, and hardware maintenance problems all come to mind as possibilities, and one blanket answer won’t cover all of them.
Interoperability, networking, and storage application layer issues will require intensive discussion with vendor sales engineers. Assuming you already have some storage products, you would need to consult with the product sales engineers of the additional vendor(s) you are considering. A test run would be ideal but is rarely feasible, so you must get in detail, in writing preferably, what will work, and what will not work together. Storage is one issue and the networking components and server layer may also be a consideration due to OS or other potential incompatibilities/conflicts. This should all be part of any RFP you send to vendors, it should be clearly detailed, and it should be examined closely for an apples to apples comparison between OEMs.
On the maintenance side, going to a third party for software and hardware support is advisable whenever you’re miixing multiple vendors because OEM vendors will generally only support their own brands and the problem will always be diagnosed as “the other guy’s problem”. Legacy or end-of-service-life become a serious concern with support direct from manufacturers but is less of an issue with third party support. Interoperability coverage is also possible with third parties but not always with OEMs directly. Some OEMs do offer coverage for other vendors products, usually by outsourcing that work to a third party provider themselves, so it may be better to contract with a third party provider directly. Due to volume, though, OEMs may still be able to get you a better price for the same service. Here again you need to ask specifically about multivendor support, legacy support, and transition from existing support providers to any third party bidder in your RFP.
Also, don’t forget to explore any risk management issues, especially if the storage is for backup, or archival data retention for specific SOX or HIPAA compliance. For that matter risk analysis on primary storage is just as important. A small glitch might be a small problem, or it might take an array down if it’s misdiagnosed or mismanaged, and storage error reports can be notoriously difficult to parse or involve proprietary laptop-appliances. This all becomes increasingly complex when multiple vendors are involved.
However these complexities aren’t themselves a bad thing, and multi-vendor storage isn’t all that unusual. The main thing is to make sure to identify pitfalls in advance and then get the real truth from the vendors about what systems or management apps their products play nicely with. Put as much of this in writing in the RFP as possible so you can compare apples to apples in th response.