UPB is the best, but as someone with maybe 50 switches, here is my advice:
1) First you said that you have many 3-way or 4-way switches, but DO NOT get tempted into using a UPB "remote" switch. They are a bad idea. In theory they can remotely control a regular switch, but there are problems. First, they don't feel like regular switches and second their indicators don't match regular switches. So instead, consider putting a regular UPB switch at each location, and connect them with a link. This gives you more flexibility because then ANY UPB can be programmed to control ANY UPB switch. In other-words, your not locked in to use set switches to control any particular light.
2) You'll here arguments here, but I recommend to stay away from Simply Automated, and instead either go with Leviton or PCS/PulseWorx or both. Here is the reason, and it goes a bit back. PCS/PulseWorx owns and created the UPB standard, which Leviton and Simply Automated licensed. All was good in the beginning, but for whatever reason, Simply Automated and PCS had problems maintaining their relationship, and again, for whatever reason, Simply Automated stopped paying licensing to Simply Automated. So what does this mean? Basically, Leviton and PulseWork switches gained Generation 2 and Generation 3 firmware, while Simply Automated is "stuck" at Generation 1. Why that is bad is because Generation 2 added support for repeaters, which Simply Automated switches do not fully support. Now you don't have to use a repeater and could use a bridge, but its really nice to have the option if you should really need it. (This is a simplification)
If you REALLY like the Simply Automated switches, contact the company Web Mountain because they buy Simply Automated switches and update their firmware to Gen 2 or Gen 3.
So long story short, UPB is the best, but with some caveats.