The Cochrane Handbook states that
incorporation of heterogeneity into an estimate of a treatment effect should be a secondary consideration when attempting to produce estimates of effects from sparse data
This has for example motivated authors of this meta-analysis to use a fixed effects model for their analysis, because their events of interest were rare.
In general, to what extent should this recommendation be followed? If I have data from studies that I have very good reasons to think that they are heterogeneous, but that report very rare events, what type of model should I use?