differential amplifier mismatch of transistor values
You're absolutely correct--a differential amplifier won't work well at all if the transistors aren't well-matched. Fortunately, transistors within an IC are quite well matched, and you can get matched transistor pairs in a single package fairly easily. The matching also relies on the transistors being at the same temperature, too, which is easy if they're on the same silicon die and much less so if they're not.
If you're willing to put in the work, I've seen discrete differential amplifiers get reasonable performance from using transistors selected to be as closely matched as possible, using a curve tracer, and then bonded together for tight thermal coupling (in the particular case I'm thinking of, the two TO-92 packages were glued together face-to-face). Even so, it wasn't as good a differential amplifier as you could make using a single-package matched pair.