Being a solo rheumatologist as your first job would be tough, but obviously you wouldn't be the first to do it. Rheumatology in particular is a specialty where it is as much "art" as science... there are many different ways to approach a particular complaint or problem and you can't always rely on guidelines to tell you what to do. In that sense I do think having more experienced colleagues who are approachable and interested in your professional development is a major plus. However, if you have a good relationship with fellowship attendings or other rheumatologists elsewhere, they can be your "virtual" colleagues to bounce things off periodically. I still text my co-fellows and fellowship attendings pretty regularly to discuss crazy cases, weird drug reactions, etc.
Beyond that, regardless of the specialty mix I think the most important thing in a new job is whether you feel like your colleagues will treat you fairly and as a fellow professional, or whether to them you are just an RVU generator that they can get rid of and replace if you don't behave. Obviously to some extent you can't know this ahead of time, but if you get weird feelings before you join the group I would pay attention to that. It's easier to back out in the first place than it is to get in a bad situation where you are being treated poorly and are unhappy. Partnership opportunities are important to know about.
In my opinion if there is no chance of partnership at all, I would be very reluctant to take such a job because you will never have any say in your practice or your future. You will always be subject to the whims and desires of either the established partners who extract revenue from your work to pay themselves a profit share, or the administrators of the group/practice who may not even be physicians and just look at numbers in a spreadsheet and tell you what to do without any knowledge of what it takes to be a good rheumatologist or deliver appropriate care to our patients.