The choices you make online, including whether to use free Instagram private account viewers, are ultimately your responsibility.
You should educate yourself on them as they are free in order to safeguard your device and your data.
Free versions could be time-limited or otherwise restricted, so you should only use them briefly. You might also be asked to complete surveys or other forms, which might be a warning flag for a dubious business.
One option to guarantee your safety is to pay for this kind of service.