From my own observations, working with outsourcing is fraught with problems.
A good outsourcing team or individual can be a godsend, taking pressure off the core team and getting extra custom content into the game that would otherwise have had to compete for project resources with other key areas.
However there can be many problems with getting to this point. Communication is the key difficulty, in all senses of the word.
Physically, your outsource partner will not be working in the same building as your team. On a practical day to day basis, this makes is harder to check on progress and keep an up to date impression of how the project is proceeding. It also makes it harder to convey to the outsourcer the underlying philosophies of the game, your design principles and the nuances of your design which influence what you want. They will also likely be unfamiliar with your working practices and your pipeline for content, possibly providing you with assets in a format that requires conversion work on your side to get into the game.
Culturally, your outsource partner may be in a different country and speak a different primary language. This exacerbates the problems mentioned in the first point. Neither are dealbreakers in any general way, and it may well be that an indie team works in a distributed and collaborative fashion anyway, but both points make it harder for everyone on the team to be on the same page and to all be working towards the same goals.
Managerially, if you have an outsourcer, you have to make sure they are managed effectively. They may well have many projects they are working on simultaneously, and not being on your "staff" you have no direct control over how quickly, or to what quality they do their work. Someone on the team will need to make time to liaise with the outsourcer on a regular basis to review progress, to discuss problems and proposed changes. One of the more difficult aspects will be signing off on a particular resource. As a separate entity, your team may have very different expectation in terms of quality from your partner. Remember that polish and final tweaks can take up a disproportionate amount of time. I have seen examples where outsource work that was submitted required extensive "touch up" work to ensure that it felt consistent with internally produced assets in game. Overall the quality was not bad, but it was different enough to be noticeable, and in the end as much effort was put into correcting this as would probably have been taken to generate the content from scratch.
Obviously, most of these points boil down to recognising the difficulties of working with someone outside your team, and ensuring good management and the agreement of appropriate specifications and goals before work begins.
Indie development is generally very content bound, small teams are usually unable to produce the quantity of content as a larger traditional team (depending of course on the type and style of game). Outsourcing, while potentially expensive and coming with potential management headaches, can provide a way to help overcome this to some extent, producing extra high quality assets for your game.