[I work at Plaid] I don't know if we explicitly write down in the docs that the free Development tier isn't guaranteed to have the same SLA as the production environment, but if you're not paying Plaid there is no SLA (I mean, the usual recourse for an SLA breach is a rebate, but you can't give a rebate to someone who isn't paying you in the first place). That said, in practice the differences between the free and paid tiers for a personal finance app are not really such that someone doing a hobbyist app for personal use would notice them.

Correct me if I'm wrong but some of the banks don't work in developer mode at all (at least, it doesn't seem to work with Chase).

All banks work in the free Development environment, but for banks on OAuth, including Chase, you need to go through the Production approval vetting as a pre-requisite. Once you've been approved for Production (and if applicable for the given bank, gotten your security questionnaire approved as well -- I think Chase requires this) you can then access those banks in Development for free.