In this game, you have a little more classes, and even if many of them are simmilar ... you can allways set them to stats ... You have tank (constitution), muscle (strength) stealther (dexterity), brain (intelligence), eyes and ears (wisdom) and finaly face (charisma) of the group ... so, in order to create "optimalized" party, you are suppose to have them all ... its ofcourse possible to focus on one aspect, instead of another, aka. specialize ... but once you have to scratch 1/3 of your options it, to put it simply, becomes frustrating.
In DnD the classic adventuring team is Fighter, Cleric, Wizard and Rogue with other classes being variants or hybrids of those 4. Even the 5e edition DM guide p.83 states that 3-5 players is the ideal party size. Also there are no MMORPG-style tanks in DnD, frontline characters are usually both tanks and muscles and face is usually either rogue, bard (who can actually be fill most other roles in 5e) or sorceror (who can substitute wizard).
Might just be my experience, but in tabletop 6 characters has always been that sweetspot. For 5e official modules I have done Waterdeep Dragonheist, Dungeon of the Mad Mage, and some of Tomb of Annihilation. Now not counting Tomb cause its meant to be hard, my 6 person group still had challenged with Dragonheist + Madmage, without any homebrewing happening. Infact with dragonheist the party was practically wiped except for my Celestial Warlock because even with 6 players the characters made mistakes and rolls were made poorly. So overall, from a personal experience I think it is completely possible to challenge a party of 6 with only RAW content.
