we started play with 5 PCs, and right now are down to 4. I can say as a DM I can see a major difference.
Our Dwarf 'fighter' at level 12 was a 10th level fighter (Eldritch knight) 2nd level paladin and then took level 11 fighter and since then has gone up wizard. He really needed that 3rd attack in order to keep up, and those 2 paliden levels came around level 5 so it hurt him more then a little. Now he is as we climb the levels focusing on spell casting more (He did just ask me if he could retrain to bladesinger from envocer)
Our 2 assasins though make quite more the case... at level 12 we had a 12th level Rogue (Arcane Trickster) and we had a Rogue (assassin) 5/ Bard (war college) 7 and out bard has never kept up to our rogue with sneak attack. The bard assassin with a dagger (adamantine +1 dagger of life stealing) doesn't even on a crit do as much as the Short bow trickster
A trick 1d6+5 (18 dex& +1 arrows)+ 6d6 sneak attack, and he just around that level got a magic ice bow that gave him the ability to make ice arrows that did 1d4 piercing damage and 1d4 cold damage and had a lingering effect of reducing regen. so instead he could deal 2d4+6d6+4 but with 1 pt less to hit...
Assassin bard 1d4+4 (17 dex and +1 soul stealer)+3d6... even with my generous reading of assassinate (I don't want to argue it again) and him getting it often in at least most (but not all) fights if he put anythought into it he would deal 'crit' damage of 2d4+6d6+4 damage.