How effective are paid backlinks these days?

Hey guys,

Quick question about SEO.
How effective are paid backlinks right now in your experience?
Before entering a niche, how do you usually measure competition?
Referring domains / DR?
SERP strength (brands, site age)?
Content quality and topical authority?
What are the main red flags that tell you a keyword or niche isn’t worth going after?
Would love to hear real results or failures. Thanks!