Bias can certainly be an issue. I'm not sure how you can avoid it completely. Is there some institution you trust more than the government? Oxford or Cambridge economists perhaps? Or being academics are they likely to have a pro-Europe bias? Podcasters?
I think the inherent unreliability of economic forecasting is more problematic than bias. On top of that, there are also some big unknowns here. Are we to assume that the UK is going to remain intact, that Northern Ireland and Scotland will not leave in five years? Will there be a world-wide recession next year and if so, how long will it last? Obviously, nobody knows, but surely these possibilities overwhelm any modeling bias.
In any event, it does not look to me as if attitudes towards Brexit depend very much on how GDP might be affected. Given the uncertainty, economic arguments are best understood as rationalizations for more tribal impulses.