Well, there are some forests and a bit of mountains in the west (though the most mountainous regions are predominantly ethnically Hungarian where you won't find much support for a Ukrainian nationalist insurgency) with a few marshes in the north, but yeah most of it is fairly open in the center and the east.

But imo the biggest obstacle is the demographics. Even before the war they had way more old people than young people, and the fertility rate was already very low, and now both of those problems have gotten much, much worse. Many of the young men have left or are already dead or injured, and the birth rates are some of the lowest in the world and with little hope of recovery for the foreseeable future. This is a very stark contrast to the kinds of populations that can wage successful insurgencies, those whose demographic pyramids are heavily skewed toward the youth and where people have a lot of children and close knit social and family bonds. Neither Ukraine nor any other European country has that anymore (nor to be fair does Russia but they are not the ones who need to wage a guerilla war). We need to constantly repeat this for the libs who do not grasp this yet: Ukraine is not Afghanistan.

It depends. The flat areas of the steppe are located in the Southern and Eastern parts of the country. There's a substantial amounts of heavily forested areas in Northern and Central areas, similar to what Belarus has (which is understandable, given that Belarus is to the North of Ukraine and has a ton of forests and swamps).

There are also mountains in the West (where the Nazis mostly come from, by the way) - they constitute a part of the wider Carpathian Belt (the Carpathians).