I don’t know with 5th edition solved spellcasters problem - them being fairly useless on low levels and supremely powerful in late ones.
High tier spells are far more limited. You can no longer have 6 wishes or 6 Stop time casts as a lv 20 sorcerer. A lv 20 warlock has only one use of tier 6/7/8/9 spells per long rest. High level spells are also much more nerfed. Finger of Death no longers OHKills. Metamagic is restricted to sorcerer BUT at low levels, casters has at will cantrips.
Eldritch Blast is the best cantrip by far. At lv2 you can pick Agonizing Blast, making your Eldritch blast into a force/charisma based heavy crossbow with unlimited ammo. So, casters are still much weaker than fighters at low level and much stronger at higher levels, but the difference is far smaller than on 3.5e.
IMO they should gave some unique abilities to martial classes on higher level, for example, Barbarians able to make people flee in terror if they fail in a intimidation check on mid combat, act like Guts from Berserker with Berserker armor, gaining DR and regen while raging, and superhuman STR/CON rangers to prepare deadly poisons that OHK on failed save or to have a huge critical range and TAME animals based on a check. Able to cripple the enemy with well placed shots also will be good.
