Not really, I've built all sorts of stuff, but ultimately when you're hooking together a client's accountancy SaaS to their order system, it's going to be rough

Not really, I've built all sorts of stuff, but ultimately when you're hooking together a client's accountancy SaaS to their order system, it's going to be roughly the same code and take the same time as the other umpteen times you've done it for other clients. But it'll be a little different as they're using Xero instead of Quickbooks which you used last, and they have a different order entity design because of X. I really feel like anyone arguing otherwise is slightly delusional. I find it particularly funny the guy two up claiming frontend might be reusable code, but backend isn't. Once you've done one invoicing/emailing/workflow/search system/time tracking/credit control/custom form designer/forecasting module/cash flow/etc./etc./etc. module for a client, you've pretty much done 'em all. And after a certain point you can then start to estimate the complexity of other systems with a small amount of talking to the client and research that let's you know the scope and the gotchas.