Solder masks are a good way to protect the antenna or RF traces, but at what cost?

The first thing to remember is that a solder mask is a dielectric with its own Dk and loss tangent, so it can potentially shift the working frequency. Lower frequencies are much more forgiving in this sense, but the higher you go the more there is to lose with a solder mask. Like in the following example - at a working frequency of the 7 GHz, a power divider shifts by 300 MHz after applying solder mask with Dk 2.4.

The best approach is to always include solder mask in the simulation as a top dielectric layer, if you are going to use it in real life. And remember to check dielectric properties - some manufacturers have a range of solder masks.