Fanaelialae
Legend
Illusions are fun at low level. Once you start fighting creatures with Blindsight and Truesight, it becomes far less fun. There are so many of them at higher level.
Whether an illusionist is strong or weak depends a lot on what the DM is willing to let him get away with (as has always been the case).
Also keep in mind that only the visual components of an illusion are ignored by creatures with Truesight / Blindsense. Conjuring a stone wall in front of a creature with Blindsense (using phantasmal force) is likely to cause that creature to veer away from it because it has a tactile component and is therefore "visible" to the creature. Causing the roar of a dragon to echo through the caverns you're in will affect (and potentially scare) even a creature with Truesight.
Illusions are best used interspersed with "real" spells, and as means of tricking the opposition into doing what you want them to do. For example, if pursuing someone, you can use illusionary walls of stone to channel them where you want them to go. They're unlikely to figure it out, since that requires physically interacting with the wall or taking an action to examine it. If they do happen to figure it out, however, make the next wall or the one after that a real one. Said someone runs headfirst into a stone wall that he believed nothing more than an illusion, and you nab him while he's reeling.