The short answer: it's not optimal, but may work. Hopefully if these welds are critical, they're being performed to some code (In the US, for most structural work they'd be AWS D1.1 and D1.2 respectively.) Aluminum is considered among the hardest metals to weld well, so quality control is especially important there. Overall, the wire feeder itself doesn't have too much impact on weld quality - a bunch of other factors including the power source, welder (person), joint preparation, consumable choice, and cleanliness of the welding environment will make a bigger difference.
Steel is an easier metal to feed through a welding lead, because it has a higher column strength, and is less likely to buckle. It's guided by a narrow tube ('liner') that runs through the cable, and is typically just pushed by the wire feeder (which may be built into the power supply or a standalone unit.) Because aluminum has a lower column strength, it is more likely to buckle, and thus not feed well. When this happens, the wire will stop feeding out of the gun, and just fill up the liner which then gets jammed and has to be cleaned out. This is a pain for the fabricator, but shouldn't affect weld quality other than maybe causing some extra starts and stops.
For a short welding lead, and using large diameter aluminum wire, they may well be fine using the same wire feeder. If not, there are a number of other strategies. For small amounts of work, they may use a spool gun which is essentially a miniature wire feeder which the welder carries around. A small spool of aluminum wire is actually inside the gun, along with the drive rollers which only have to push the aluminum a few inches as a column before it gets to the weld. Another common strategy is a push-pull gun, where the wire feeder pushes the wire with to help it come off the spool, but additional drive rollers in the gun pull it to keep some tension on the wire as it goes through the liner. It is possible that they are using the same wire feeder, but changing from a conventional MIG gun to a push-pull gun when they switch from steel to aluminum. That would be a common practice.
At the end of the day, I wouldn't be worried about their choice of wire feeder. For the most part, it will work or it won't. I would be more concerned with their overall quality control system. Are welders working to a written procedure? Have welders been tested before they're set loose to weld? How and how often are finished welds inspected? If they are welding to a code, these answers will be dictated by it. If they are not welding to a code, you can get a rough idea of what level of QC they employ.