There's so much variety in hybrids that's hard to discuss as a single category.

I think the most important question is whether the system requires a regular automatic/manual transmission or forgoes one entirely. The Toyota planetary gear system forgoes one, as does the modern Honda and Nissan approaches. Not having a transmission in the traditional sense saves so much complexity that the overall system is net simpler imo even with the additional complexity from having a motor and engine.

Then there are systems that have a full automatic drivetrain and some extra clutches to couple to a motor-generator. And there's even systems with an electronically controlled manual transmission instead. Those systems are going to be incredibly complex and fragile.