Why does not one talk about Solo 401(k)?

[Kase Knochenhauer]

I considering opening a solo 401(k) for our new long term rentals. 

Why aren't more people talking about this and instead refer to a Self Directed IRA instead (even though it seems leveraging a Solo(k) offers more opportunity, larger annual limits, and lower operational costs).

The company we are looking at is solo401k.com since they charge only $399 to open and a fixed $99/annual fee.

Anyone have experience with this retirement account or company?