Plus truvada resistant strains of hiv have been recorded.
Also "blips" i.e. Those times when a viral load becomes detectable between periods of undetectability: happens to many, if not most, hiv positive people on drug therapy. Much like when the hiv test was first developed and you were only negative as of the day of your last test, or really a few weeks before given the incubation period and the slowness of the test back in those days, so too with "undetectable" you are only undetectable as of the date of your last labs. To further compound the situation, labs are supposed to be done every 3-4 months, so to lay a framework you can have the following results over a year or so : period 1 undetectable, period 2 undetectable, period three detectable, period 4 undetectable . Now it is very possible that between period 1 and period 2 labs a viral load would become detectable then revert to undetectable by the time of the period 2 labs and there would be no way of knowing that sequence of events.
PREP without condoms is still a gamble.