I should have been more precise - I should have said probe flight times. The few early Soviet Mars probes that survived launch died on the way before they could reach the planet. For Venus transit the whole thing needs to keep working for much shorter time before descent module lands and the mission is over in minutes, as you mention.
It doesn't seem that transit-phase failure was a primary cause of incomplete Soviet Mars missions, though several did fail after systems (mostly electronics) malfunctioned during coast phase. Several failed in boost phase, or in leaving Earth orbit. Two of seven missues suffered from electronics degradation which either resulted in a total or partial failure to reach the Martian surface. Mars 2 & 3, both soft landers, failed for other reasons (navigation, and perhaps Martian meterological conditions), though both arrived at the surface. Mars 2 perhaps rather faster than desired.
Mars 4 & 6 both seem to have suffered transit-stage electronics failures.
https://en.wikipedia.org/wiki/Venera