One of the best features of the Hillebergs are their ruggedness although I can't vouch for their new 3 season line. I believe the TarpTents have thin 30 denier floors on most if not all of their models though I could be wrong about their larger tents like the self-supporting Scarp 2 or their Hogback. My Hillebergs have 100 denier floors which is like the difference between a pair of blue jeans and a pair of silk underwear.
As Mags says, here in the Southeast we get some terrible combinations of rain with cold or sleet or fog. Last year I pulled a trip into the Snowbird backcountry and got caught in a 153 hour rainstorm in January/February. My previous record was 96 hours of rain. The problem with many of the ultralight tents are their, of course, thin fabrics and floors. Here's a couple tests---
** Set up in a rainstorm and prepare to get either ground sheeting or Lake Effect---whereby the tentsite fills with a half inch or inch of water. A good tent will keep this crap out. And in a butt hard deluge no campsite will not get water runoff.
** Now, put all your weight on the tent floor and see if water comes up thru the floor like a sponge---due to weight pressure and poor hydrostatic head, a fancy word for thin deniers and water absorbtion. I like to stay dry in all conditions, at least while I'm in my shelter.