Yes in the sense that you would need some time in the hospital with acute care to recover from all your systems shutting down, and probably not in the sense that you would have permanent damage. The main fear would be brain image from oxygen deprivation and it doesn't seem to have been observed - of course, low sample size due to the few failures, but pentobarbital may also protect neurons from some of this damage (there's no real conclusive consensus on how well it does, that I can see). It's a risk you incur with any depressant overdose though; people who ODed on benzos and opioids have shown cognitive injury in at least one study conducted after discharge.
As far as overdosing goes, or any method, it's extremely reliable and the damage you risk by failing is pretty minimal.
If you're scared about this, btw, leave something that makes it clear what you used. Overdosing on N is rare, so it'd be easier to treat accurately if they were aware it was pentobarbital. But it should be lights out as long as you aren't found and you take enough. (I saw you mention in another thread sending a youtube live link to somebody - don't do this if you don't want to be found - it may have been dark humour or venting, I don't know, but streamed suicides usually get the cops called.)
