The Car Care Nut answers the question on this video. He's an experienced Toyota and Lexus technician and his answer is that they tend to be very reliable so long as the air cooling works adequately.
Overheating is one of the most detrimental things that can happen to a high-voltage battery in a hybrid system. The Car Care Nut also points out that as the intake of the air cooling system of the hybrid battery is inside the cabin, the owner must remember to keep the filter clean. In some Toyota and Lexus models, the filter is easily accessible like in the case of the Lexus RX where it's behind the front seats close to the floor. In the Lexus GS 450h and the 300h, the air intake in the door jamb. The filter in some model years in the GS is not close to the air intake and thus much harder to reach and thus probably a service item. The fact that the air intake is inside the cabin is an excellent reason to keep the cabin of your hybrid clean and free of dust and lint. A high-voltage battery is an awfully expensive thing to replace. In most models, the batteries cost between $3000 and $5000 labor and taxes included. IIRC, the Lexus LS 600h uses a type of high-voltage battery that costs $8000.
Below are repair statistic from Finland (English subtitles of readable quality by auto-translate available). The figures include batteries replaced due to accidents. For this reason, the statistics understate the durability of the batteries, which is actually very good. Finland has a cold climate, which this statistics may reflect to a degree.
Toyota has nailed it when it when comes to the Toyota Hybrid System. The eCVT transmission, which has nothing to do with conventional variator based CVT systems, is mechanically very simple. The variable speed ratio between the internal combustion engine and the driveshaft is achieved by simply varying the rotational speed and direction of the sun gear in a single planetary gear set. In the eCVT transmission, the internal combustion engine is connected to the planet carrier and the outer gear is connected to the drive shaft to which the larger one of the electric motor-generators is connected, too. The smaller one of the two motor-generators rotates the sun gear.
Here's a video that shows exactly how it works:
There is very little that can fail mechanically in this setup because oil pressure is not used to do any physical work unlike in a torque converter. The transmission oil flow in inside the transmission housing it isn't actually even pressurised as it of the slush type. The electric components rely on a separate liquid cooling system. Everything is mechanically connected at all times and in all driving modes.
Toyota was the first manufacturer to introduce the hybrid drive train and maintains the lead in reliability.