I really dislike the SAFE text, but leave that aside. I've done numerous CLA agreements that were a page or two; meaning, we negotiated them in less than a dozen hours. If you're two parties who want to close a deal within a week, then a CLA will work. A full equity transaction is still a mess, it's still a lot of paperwork and a lot of files that need to get organized.
Just think of the IP binder, if you're a good startup you need to map your open source code; you need to sign all the employees again and you need to start organizing the cap table.
I really prefer CLAs for anything; both when I'm representing the investor or the company.
RE: CONVERTIBLE ROUNDS ARE NOT FASTER AND CHEAPER