Eric makes good points--and we always had two large EPIRBs on our long distance boats (one with the ditch bag). However I am not aware of at least the ACR PLB having a problem with the GPS antenna being obscured by a holding hand.
The fast find should be GPS enabled--they are just as much as EPIRB as the larger units--same frequency and power, but shorter battery life. Hopefully you will have a good life jacket on if you go into the water--and holding the EPIRB/PLB will not be a problem.
The major advantage of the Fast fine or other PLB, is that it is WITH YOU ALL of the time. There are many instances of deaths where there was an EPRIB on the boat, and there was a rapid sinking where the ERIRP was not pulled out of its holder and the people suddenly in the water had no way to give a distress call. The PLB stay in our pocket/pouch, along with the waterproof VHF, Strobe and whistle--plus it goes on the dinghy, Kayak as well as on land. It is always available. When deployed the unit shoul.d be tethered to your jacket--and it will be with you. The free floating EPIRB may drift away. The purpose is to save your life, and if you don't have the rescue device with you--you may be cooked. This is the reason I have PLBs (and if going offshore, will again carry two EPIRBS).