The absolute biggest drawback here is that you can't leave optional-PvP once you transfered there. That means, if you do regret it later on, there's no way back.
I think it all boils down to the reasons as to why you're moving to optional. What is it that you want on an optional that you can't get on open?
Keep in mind, optional PvP doesn't mean less abuse from other players, probably the opposite, as some players will find extremely annoying ways to ruin your gameplay without having to resort to PvP and there's a lot less you can do to retaliate. The recent exiva-changes, that made it possible to block people from exiva you, made it a bit easier to hide from "dominando" players, but the problem of Optional PvP dominando is still very much a thing (and will probably remain so until Cipsoft decides to help out).
The servers also tend to be a bit more populated than most Open PvP worlds, aside from the obvious popular ones such as Antica for example. This means that houses can get a bit more rare and expensive and the tibia coin price tend to be a bit higher too.
That said, as an optional PvP player since many years back, there are days I wish I did play on an open pvp server, but I never regret starting optional. The community on the optional PvP servers that I have played have been extremely welcoming and nice to me.