I suppose it depends on the specific laptop you are considering. Most fanless laptops use Intel Core M CPUs or the less powerful Intel Pentium, Celeron or Atom CPUs. So you are giving up performance for a fanless design.
However, there is one laptop (actually a 2-in-1 convertible) that has a uses liquid cooling to cool the Core i5-6200u or Core i7-6500u; the Acer Switch Alpha 12 which competes with the Surface Pro 4. The liquid cooling works well enough where there is no thermal throttling. Links below to a video review and written review of the Acer Switch Alpha 12.
https/www.youtube.com/watch?v=wP404GpoE2Y&t=380s
http
/www.mobiletechreview.com/notebooks/Acer-Switch-Alpha-12.htm
Acer has announced the Switch 3 and Switch 5 which replaces the Switch Alpha 12, but they have not been released yet. Both will be using liquid cooling. The Acer Switch 3 will have either a Pentium or Celeron CPU and is expected to start at $400. The Acer Switch 5 will have either the Core i3-7100u. i5-7200u or i7-7500u with an expected starting price of $800.
https/liliputing.com/2017/04/acer-switch-3-apollo-lake-2-1-coming-soon-399.html
https
/liliputing.com/2017/04/acer-switch-5-2-1-tablet-coming-june-799.html