Just had the most agonizingly atrocious game earlier. Opposite team had what was clearly a 6 man party and they were extremely sweaty tryhards. Really good sniper with a vac-med, heavy with a pocket medic and a sticky demo with a pocket medic. I was playing on Upward and we couldn't even get out of spawn. Defenders ended up winning without us being able to capture the first point. Demoman on the defending team ended up with 11 dominations and nearly 200 points. The whole party system is so ass and was clearly only made for competitive.
You can argue it's just a skill issue, but it's almost impossible to stop a dedicated pubstomper 6 man party on casual.