Multiple martial classes multiclass really, really well together. There is a single dead level at the 5th level of the 2nd multiclass class, but for the most part, all of their other mechanics add together (or better, multiply).
The "you don't get access to actual higher level spells" makes anything but shallow multiclassing of full casters usually suck. You still get the slots, but not the spells.
Many melee classes are front loaded; level 1-4 gives more toys than levels 6-9. So grabbing 1-4 levels of another class gives you lots of toys to work with. Then the dead level at 5 makes reaching 6 overly expensive, even if the feature is nice. So you only level one of your classes past 5. The earliest point where you make this choice is 6/6 or 4/8, and the level 7 and 8 features of class A are usually better than the level 6 feature of class B.
In practice, characters are in the level 3-12 range; few people play high level D&D. Level 20 builds are usually white-room. Going level 5 (extra attack is an extra-large feature) then dipping another class for some interesting features becomes very tempting. So now you have a plan to level 9. Going from 9 to 12, your choices are 4/5678 or having a dead level.
Going 01, 11, 12, 22, 23, 33, 34, 44, 45 is painful, because you wait so long for that extra attack feature. But, a 2/3 with extra attack is probably going to dominate a 5 pure class, as they have gotten 5 levels of features while the 5 pure class got 4, plus extra attack.
One really crazy idea is to skip the extra attack level on the feature track of your second class. You just start getting level 6 features at level 5 in that class, etc. You can also permit someone who already has 4 levels of "extra attack class" levels to grab extra attack on its next level up in an "extra attack class", delaying that classes non-spellcasting features by 1 level until it catches up at the extra attack level (if ever).
The "you don't get access to actual higher level spells" makes anything but shallow multiclassing of full casters usually suck. You still get the slots, but not the spells.
Many melee classes are front loaded; level 1-4 gives more toys than levels 6-9. So grabbing 1-4 levels of another class gives you lots of toys to work with. Then the dead level at 5 makes reaching 6 overly expensive, even if the feature is nice. So you only level one of your classes past 5. The earliest point where you make this choice is 6/6 or 4/8, and the level 7 and 8 features of class A are usually better than the level 6 feature of class B.
In practice, characters are in the level 3-12 range; few people play high level D&D. Level 20 builds are usually white-room. Going level 5 (extra attack is an extra-large feature) then dipping another class for some interesting features becomes very tempting. So now you have a plan to level 9. Going from 9 to 12, your choices are 4/5678 or having a dead level.
Going 01, 11, 12, 22, 23, 33, 34, 44, 45 is painful, because you wait so long for that extra attack feature. But, a 2/3 with extra attack is probably going to dominate a 5 pure class, as they have gotten 5 levels of features while the 5 pure class got 4, plus extra attack.
One really crazy idea is to skip the extra attack level on the feature track of your second class. You just start getting level 6 features at level 5 in that class, etc. You can also permit someone who already has 4 levels of "extra attack class" levels to grab extra attack on its next level up in an "extra attack class", delaying that classes non-spellcasting features by 1 level until it catches up at the extra attack level (if ever).
