Most of the concern will be in 2 areas. How well the systems were designed, and how well they have been maintained. If you can't verify one or both of these, then you could be signing yourself up for significant cost burdens. In addition, since the well is providing drinking water, you need to know something about the water source they drilled into.
There are companies that can do inspections of these systems, but sadly, most inspections are not detailed enough to answer either of the 2 concerns above.
Your best bet is buying a home where these systems were installed within the last 10 years or so. Restrictions implemented in many jurisdictions in the recent past, substantially increased the reporting requirements for installations, and some even require third party maintenance contracts.
Note, there are many, many homes sold with these systems that operate just fine, but you need to know the potential costs to repair these if they go bad. While locale will make a difference on costs, consider that a new septic pump, including installation, will run $500 to $1,000. A new septic tank (if one is cracked), could easily exceed $3,000 depending on size. A new septic field (where the gray water drains to) can easily exceed $10,000 and cause quite a mess of the yard. And if you had to replace the entire system, you might have to comply with current restrictions on design, which can tack on additional costs.
For a well, it depends on how deep it is and what kinds of problems you run into. For example, if the pump goes out, you should expect to pay at least $1,000 including installation. If the water dries up and they have to drill deeper, who knows what the cost will be, but it ain't cheap. If the water goes bad, well, you're toast.
But again, a lot of homes are sold with these systems without any problems. You just need competent local resources to help you with the decision.