Adding Unity integration to your freelance offering sounds like a natural extension of 3D modelling work. And it can be genuinely profitable - clients are willing to pay more for someone who delivers a working prefab rather than a raw FBX. But the scope of that work varies enormously depending on the project, and underquoting it is one of the more common mistakes freelancers make in their first year of game work.
The case for offering integration services
Unity's import pipeline is well-documented and relatively forgiving. The Universal Render Pipeline has standardised a lot of material workflows, and for straightforward static assets - environment props, UI elements, modular architecture pieces - integration is usually 30 to 60 minutes of work per asset once you know what you are doing. Clients who find a freelancer who handles both modelling and prefab setup tend to stick with them, which means more consistent work over time.
Pros for freelancers
Higher per-project rates are realistic. Fewer handoff errors because you control both stages. Easier to build long-term client relationships when you are a single point of contact for the full asset pipeline.
Cons that affect your bottom line
Animated characters, physics-enabled assets, and anything with custom shaders are a different category of work entirely. A rigged character with blend shapes and a custom LOD setup can take 6 to 8 hours of integration work alone. If you quoted that as part of a flat modelling rate, you have already lost money before the client signs off.
The fix is straightforward: separate your quote into modelling and integration line items, and specify exactly what integration includes. Define whether you are delivering a prefab with materials applied, or just the imported mesh. Ambiguity always resolves in the client's favour, not yours.