My perspective is that of a serial LP investor in MR RE syndications. I used IRA funds one time and regretted it. Luckily I redirected most of my 401K would be contributions (match only) to non-qualified account then jumped into RE.
There are some parameters that need to be set. First, I assume anyone considering this will be wealthy in retirement, ie. your tax rates will not be low at that time. Second, ALL funds in traditional 401K/IRA funds will get taxed and paid by someone! Maybe your heirs, but there is no way around it - recall I assume you will not be sliding by with no income. Third, note that any RE done in a traditional qualified account (SD-IRA, Solo 401K, etc) takes all that wonderful Cap Gain income and turns it into earned income. Also if owned outside qualified accounts, that depreciation would defer income, in fact, if you consider a 1031 (available to direct owners, not so much to me in syndictions) you can keep exchanging and your heirs get a step-up in basis; the tax on the gain is never collected. Fourth, getting funds out of a 401K is a bit tricky; typically you have to quit, you might get an in-service distribution, other? Fifth, Roth vs Trad does make a difference.
As indicated above the tax will be paid, it falls out of the equations. $1M in a Trad IRA is really only $750K if your tax rate is 25% and you are already above your std deduc, etc. Do the math, pay taxes now on a small amount or pay taxes later on a larger amount, it really doesn't matter - this is very true when making the Roth Conversion decision and is pretty close on distributions.
The issue is the penalty! There are two ways around the penalty, but both take time. You can convert funds from an trad IRA to a Roth IRA, 5 years later the converted amount can be taken out penalty free. If you are say 50 or so, the 72T will allow you to take substantially equal amount out each year until 59.5. Note that if you are 55 in an 401K and leave work, you can access those funds.
I know folks that have pulled the trigger and just taken it all out. I know folks that have borrowed from 401Ks. Both groups have been happy they did. But not I hang with successful RE folks. If it blows up, you will not be happy! The lesson here is to learn how it works and the risks before you jump into RE.
There are issues with using qualified funds in RE. You must not comingle! Both trad & Roth IRAs are subject to UDFI tax (surprise!). Solo 401K is not.
As mentioned above all trad accounts come out as income in the future. Study the affects of future RMDs on your taxes and fees. These will come back to bite you! Look up Medicare's IRMAA, Social Security tax rates (no idea if you will see this, but it will likely be around is some form), and the Widow's Tax Trap, which will cause the remaining member of a couple to pay so much more in taxes.
My point here is that deferred income may seem nice now, but you will come to hate yourself when the taxes come due.
Regards!