5 is fine tbh
Engineers are not that hard to displace. One demo can easily take out 3 of em in an uber. You just have to rally the team to push together with you. Uber up front rest in the back.
I have done it plenty of times, you just have to motivate the rest of the people to join in on the push. (Sentries only target the closest by player so the uber up front would mean they get all the flak)
You only need 1 competent pyro + medic to completely ruin a demoman uber. Thats much more likely than to get a whole team to push at once.
It also depends what time of day you play, because as everyone should know, in the morning the random players are normally pretty incompetent
Anyway, i'm not finding that the engineer limit is the problem, its the class immunity which means we have 80% of the team as snipers, spies and engineers. Even with a demoman uber to take down the sentries a team mostly consisting of spies and snipers isn't going to be very effective (Unless the other team has the same problem, then the game just becomes silly

).