It's even worse than you paint it, for a lot of adjuncts. Besides the lack of benefits and insurance, often they do not actually know their course allocations in advance, and can suddenly end up losing 1/2 their income or whatever, with a week or so's notice.

For comparison if you are full time faculty at a research (i.e. R1) oriented department, a 1-1 or 1-2 (e.g. 2 - 3 courses a year) is typically considered a full time teaching load on top of your research, although you often have a graduate seminar or two on top of that. If your department focuses on teaching, a 4-4 load is typical. "Teaching relief" is given to new and productive research faculty. Summer courses are often taught for extra income, and/or can be applied to soft salary. Similar things mutatis mutandi if you aren't on a semester system.