If I remove restart-always I get some more information about the errors. One was about the audio buffer: I added '-z 940' option and it disappeared.
I still get a warning about the periods: # systemctl status -l fluidsynth.service fluidsynth.service - FluidSynth Synthesizer Loaded: loaded (/etc/systemd/system/fluidsynth.service; disabled) Active: inactive (dead) set 28 22:27:53 fede-xps systemd[1]: Starting FluidSynth Synthesizer... set 28 22:27:53 fede-xps systemd[1]: Started FluidSynth Synthesizer. set 28 22:27:54 fede-xps fluidsynth[1828]: fluidsynth: warning: Requested 16 periods, got 8 instead set 28 22:27:54 fede-xps fluidsynth[1828]: FluidSynth version 1.1.6 set 28 22:27:54 fede-xps fluidsynth[1828]: Copyright (C) 2000-2012 Peter Hanappe and others. set 28 22:27:54 fede-xps fluidsynth[1828]: Distributed under the LGPL license. set 28 22:27:54 fede-xps fluidsynth[1828]: SoundFont(R) is a registered trademark of E-mu Systems, Inc. set 28 22:27:54 fede-xps fluidsynth[1828]: Type 'help' for help topics. set 28 22:27:54 fede-xps fluidsynth[1828]: > But I guess this doesn't explain anything. Any systemd expert here?