I tried one in the carpark and it was hideously uncomfy
This made me lol a little too.
There's such a range of adjustment that can be made to a bike provided the frame size is a reasonable choice. Even if you get one that's comfy now, over time you can swap the stem (different length, rise, drop), change the bars (shape, width), fiddle with the stack height, saddle position (fore/aft) and height, change the saddle for a different shape, shifter positions, crank length. You'll be lucky to find one where everything is right for you off the peg though. And all these things you will probably want to tweak over time too if you are new to it all.
Though given there is so much scope for adjustment, this is probably not bad advice...
Ignore them and buy a comfy bike.