It is lightweight, very efficient and can be integrated in WordPress as well. When organizations want to optimize their content online, Squirrly SEO is probably the best option. It shows you exactly what needs to be improved on your page and makes suggestions. Moreover, the Focus Pages feature scans your pages and provides you with a probability of your ranking for the specific keyword.
Semrush's content and page optimisation tool is quite poor; it's not in the same bracket as Surfer. Frase is a good alternative from a cost perspective (if not paying the AI writing functions), it's cheaper, and Frase's keyword/topic heatmaps are quite unique. Sufer has a better UI and a shorter learning curve. It has an advantage in that it offers internal linking recommendations, which Frase lacks. Surfer also has limited competitor analysis (backlinks and page speed), which Frase does not. Both tools allow you to choose the number of competitors on the SERPS