The primary concern I would have is that you are taking a collection of diversified assets and betting them all on a single asset class. Whatever the "X" stands for in the sentence: "I'm taking all my cash, IRAs, and 401k accounts and putting it all into X" is a guarantee you're about to embark on the financial version of Mr. Toad's Wild Ride. The first principle of asset management is avoidance of concentrated risk.
Whether you should liquidate your retirement accounts to avoid a mortgage is a separable issue. Consider this, equivalent, situation: you own a house outright, and have a chance to take a 100K mortgage loan, have to government top it up, gratis, by 28% or so (insert your tax rate here); you can invest it in an account that protects it from all taxes for 30 years. Oh, and the government subsidizes your (30-year fixed!) loan, too, so the effective fixed rate is 4% rather than 6%.
