It seems highly probable that other Boeing models have serious safety issues, because the problems with the 737 MAX 8 are very clearly problems having a systemic origin in Boeing's engineering management culture. See Boeing 737 MAX Debacle Continues.
The problem was likely caused by mechanical linkages between the hydraulic actuators in the tail elevators. These had rivets designed to shear in the event of one actuator siezing, allowing rhe two remaining actuators on that elevator to control it. In the wreckage of flight 990 all five of the actuators recovered had sheared rivets. At the time of the investigation, the NTSB said that this could have been caused by the impact of the aircraft as it hit the water. However .... At 41 mins 53 secs. "In January 2014 the FAA issued a new air-worthiness directive that showed that a problem did exist with 767 elevators. It ordered the replacement of all bell-cranks that still had shear-rivets, with newly designed bell-cranks that did not."
This raises the question as to what other measures were put in place to allow the elevators to be controlled in the event of the seizure of one on more of the hydraulic actuators. This is material to the 737 MAX 8 as well, because some eye-witness accounts of the Ethiopian Airlines crash earlier this year mentioned smoke issuing from the tail of the plane just before impact.
Four months ago a Boeing 767 crashed near Anahuac, Texas. This was also a sudden nose-dive caught on video:
Here's the CNN report:
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