Integrated (or non-removable) liners are built right into the boot shell, so you can't pull them out like you can with most snowboard boots. You'll typically find these on some youth models and entry-level boots—they're a cost-saving design that also keeps the boot a bit lighter.
The real downside is day-to-day convenience. After a full day on the hill, you can't take the liner out to dry it properly, which means moisture and odor can build up faster. And if the liner packs out or wears down over time, there's no replacing it—you'd need whole new boots.
For kids who outgrow their gear quickly or very casual riders on a tight budget, integrated liners can be perfectly fine. But for most riders, especially if you're out multiple days a season, a boot with a removable liner is the smarter choice for the drying and replacement flexibility alone.