This year I got a client who had a solid Shopify but was angry with them and determined to be on Big Commerce. He had 40K skus with 60GB storage at the time. I got him moved over, got everything working OK again then Big Commerce was not working for him. So we moved again to Inmotion. Got it setup, again it wasn't working for him. Moved again to Greengeeks, got that working for him — and currently finally hitting our stride.
But that was a nightmare. I'm sure I shaved 3 years off my life doing these moves. Getting everything working right then hitting a wall before you can have a sunshine period is demoralizing. Doing it 3 times in 9 months can have you questioning a lot of things.
why wouldn't you be able to find out what happened? Do a comparison using the new date range where everything plummeted against a previous date where things were humming along. Go to pages and sort by click/impression differences to see the largest negative numbers. Those are the pages that dropped. Hone in on those and click queries to figure out what queries are no longer driving impressions/clicks. 4 minute analysis to find out where you lost. Fixing it may not be an option, but maybe it is.
My biggest nightmare now is I will be hit again with 10K of Spammy links. It worked great for the Ass that attacked my two sites, traffic was halved on the two sites.