My verdict: the reference SERP API for critical projects.
SerpApi is more mature and reliable than alternatives. The safe choice for production. The reference API for scraping Google and other search engines. SerpApi is more mature and reliable than ScaleSERP, with better coverage (Google, Bing, Yahoo, Yandex, etc.). The API is excellent, the documentation is clear, and reliability is there. Pricing is per search: starting at $50/month for 5000 searches. Slightly more expensive than ScaleSERP, but more stable. If SEO is critical for you, SerpApi is the secure choice.
What I like less: the slightly higher price than alternatives. For non-critical projects, ScaleSERP may suffice.
My advice: if you're building a product or tool that depends on SERP scraping, invest in SerpApi for reliability. For personal monitoring or less critical projects, ScaleSERP is a valid and cheaper alternative.