Redirects are the load-bearing structure of any migration. Every old URL that had links, impressions or traffic needs a one-to-one redirect to the closest equivalent page — not to the homepage, which search engines treat as a soft 404 and which loses the signal entirely. A migration plan that redirects everything to the root is the single most reliable way to lose a year of SEO.
Chains and loops are the quiet cost. Each hop wastes crawl budget and dilutes signals, and chains accumulate over the years as one redirect is layered on another. Audit them periodically and rewrite every chain to point directly at the final destination.