"Should startups scale with online ads?"

Yes, but only after you exhausted:
1) Direct Sales potential;
2) "Word of mouth" potential;
3) Content Marketing potential;
4) Partners Program potential.

In other words, when you have built an organic base of your marketing channels.

If you already done with it, measure exact values of customer acquisition costs and try to keep it. It's normal, if paid channel has a fluctuation about 20-30% to this value, but don't exceed this figure.

One more thing, don't focus your attention only on CAC. Make cohort analysis for paid channel, keeping high quality of lead nurturing.

Remember, that Quality always outweighs Amount. Mostly because of viral and organic potential of quality leads.