We speak of austere morality, rigid rules, rigorous discipline, stern commands, severe punishment, harsh speech or a harsh voice, hard requirements, strict injunctions, and strict obedience.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

serious Loss of health is more serious than loss of money.
critical I can't understand why you are so critical of him.
strict Algeria has to relax its strict economic policies.
cruel I think it is cruel to trap animals for fur coats.
austere The Wadden Sea is a landscape of austere beauty.
unembellished The lie sounds good, but I'd like to listen to the unembellished truth.

Popularity over time

Word	1850 → 2019	Trend
severe		steady
serious		steady
critical		rising
strict		steady
cruel		rising
austere		steady

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: bad¹
4 letters: hard¹, cold¹, bare¹, sore¹, dire¹, dour²
5 letters: short¹, heavy², tough¹, sharp¹, plain¹, grave¹, harsh¹, cruel², acute², stark¹, crude¹, stern¹, stiff¹
6 letters: strong¹, simple², strict¹, brutal², modest², wicked², chaste¹
7 letters: serious³, extreme², violent³, intense², painful², drastic², spartan², austere², ascetic³
8 letters: critical³, terrible³, rigorous³, grievous²
9 letters: difficult³, dangerous³, unadorned³, unsparing³
11 letters: unvarnished³
13 letters: unembellished

The small number after each word is its number of syllables (dour² = 2 syllables).