To be fair, BTL isn't that bad, at least it's A LOT better than FMC Fury campaign (though I can't really complain something free )
Actually, in term of difficulty BTL is at least on par with Red Hammer. Since you play as resistance fighter, there aren't much arsenal on your hands. Most of the time you have to avoid frontal battle and fleeing is your last thing can do. These are somethings official campaigns generally lacks. Some mission objectives are quite creative indeed. Overally it's not up to the level of offical campaigns, but not far away.
BTW, I've tried ULTCO mission pack too, very impressive. It successfully gives you the feeling of being in the army. Most missions are very well written. The only shortcoming is some missions don't end due to deserted soldiers or triggers which never happen. Luckily (ironically) it's only avilable as single missions so you can simply skip them.
For anyone who are interested in FMC Fury campaign and ULTCO mission pack, head for Avon's lady FAQ to have a look. I think she is still hosting them.