I've had windows replaced in all but one home I've had, and the standard of fitting has been poor even with Fensa or the other methods now permitted. In every case the gaps around windows were hidden with trim (very little foam filling etc) and so for my current home I went with no trim on the inside when replacing the doors and windows so I could make good and have a no trim internal finish.
Even with some gaps visible the Fensa company I used left huge gaps to the brick and relied on the outer silicone sealant under the external trim. They only used a little cheap (crumbles) foam in a few places where the gaps were too big to hold the sealant. This meant air from the eaves, cavity etc could whistle in.
This explains why previous houses with new glazing often had nasty drafts once any internal trim cracked its sealant, which is common as the trim is glued to the window and uPVC does expand/contract. In the current house some plaster had blown around the previous replacement windows with tiling outside (so no effective external sealing at all) and the damp air getting to it was the cause.
So for the new windows I took the opportunity to hack away some plaster, repair excessive brick/block edge damage, fill the remaining gap with a high grade acoustic flexible foam and then plaster/fill as required. The difference was staggering and the thermal camera and sound meter tests I did before and after confirmed the vast difference both with the new windows and also with a couple that were replaced to match even though they were fairly recent and had trim both sides.
In the cold snap earlier this week I did another external thermal camera check and when I compare with the neighbours houses it makes the effort worthwhile. Windows are sold on the thermal and acoustic qualities measured in a lab, but in practice they're often fitted rapidly to save costs and hence with little care about the gaps around the edge and how that impacts performance even with internal trim.