During testing, the budget-friendly smart scales were reasonably accurate. Of course, there's always some variation between models, but the relative accuracy (measurements on a single device) is the most important factor to consider, as this will help you keep track of changes.
Generally, the main reason that the scales are cheap is that they lack the features of the more premium models. So, some don't come with Wi-Fi and only sync via Bluetooth, or don't have multi-user support.