I personally have never seen an UFO actually spawn. However, new ones are occasionally spawned / built. Sometimes, when I started another raid on the other dimension, new UFOs were up and about, though it usually was only one (most frequently an escort ship, which is probably not the best craft to build if you have nothing to do damage
). Incidentally, I prefer to fight through the alien dimension before letting biotrans anywhere near it. I first went in there with one Annihilator and a Retaliator and the biotrans... the biotrans decided to arrive first and limped back home while the other craft were still arriving. The fighters took a heavy beating, too, and it took several runs to get some breathing space around the portals. (Battle of Britain SciFi edition...)
Ships: I first played on medium and used heavy armament most of the time, i.e. I used plenty of Hawks and then switched to Retaliators (Missileboats with one stasis launcher) and Annihilators (Gunboats).
In my current game (second hardest difficulty) I command a fleet of bikes and some phoenixes mixed in between, one Hawk for ... eh .. demolitions tasks and the one and only Valkyrie one as a backup skyranger (usually my soldiers drive around in a wolfhound but since I get loads of missions at once most of the time the wolfhound just drives to the nearest one. The valkyrie then picks up the soldiers from there to move to the next hostile site provided the survivors are up to another mission)
Anyway, I'm very happy with those bikes, also I loose one or two during most of the heavier alien assaults they're cheap and easy to replace, the only problem is the enormous amout of elerium 10 - 15 lineage bikes use when they're berserking through the city.