For Americans with access to them, workplace 401(k)s can be the simplest type of retirement account. After all, signing up for one just involves doing a little paperwork and requesting to have money withheld from your paychecks. And in some workplaces, the default is to auto-enroll you, so you really don't have to do much of anything. Many employers even offer matching funds, so you get access to free money by contributing.

But while you definitely want to put at least enough into your 401(k) to earn the maximum employer match, using a 401(k) as your only retirement account may not be the best move. In fact, there are some big pitfalls to putting your money into this account exclusively. Here are three of them.

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Most 401(k) accounts offer a very narrow pool of investment options. You're usually restricted to a few index funds or target date funds and cannot purchase shares of individual stocks. If you invest some of your money in an IRA, however, you'll have access to a much wider pool of investments -- any that your brokerage of choice offers. You can even open up an IRA that allows you to choose non-traditional assets, such as gold or bitcoin.

While you take on more risk by buying shares of individual companies -- and alot more risk by buying some of those non-traditional assets -- you also have the potential for much higher returns than if you stick to just the funds your 401(k) provides. If you're willing to branch out and put in the time to find the right investments, it's definitely worth putting some of your retirement money in accounts that give you more freedom.

Some 401(k) accounts have management or administration fees that eat into your returns. The funds your 401(k) allows you to invest in may also carry higher fees than others that might be available with the broader range of choice a brokerage firm provides.

Fees can make a measurable impact on the amount of money you end up with in retirement. If your workplace plan has them in abundance, you're doing yourself a real disservice if you use it as your exclusive retirement account. Instead, in this situation, you should only contribute enough to get the match and then look for a broker that offers a no-cost IRA with a choice of affordable investment options.

Distributions from a 401(k) are taxed at your ordinary income tax rate. Distributions from a Roth IRA or Roth 401(k) are not taxed. If you have access only to a traditional 401(k) at work and use it as your sole retirement savings account, you'll owe more in taxes in your later years than you would've if you'd put some of your money into a Roth IRA and taken tax-free distributions to help support you.

Social Security benefits also become partially taxable once your income hits $25,000 as a single tax filer or $32,000 as a joint tax filer. But not all income is countable. Your 401(k) distributions are included when determining if Social Security benefits will be taxed, but distributions from Roth accounts aren't. If you put all your retirement savings into a 401(k) and withdraw enough to cross the threshold at which you owe tax on Social Security, you'll owe even more money to the IRS that you wouldn't have if you'd received some Roth income instead.

Since you could reduce the size of your nest egg by getting hit with fees and limiting your investment options and get to keep less of your money due to higher tax bills, sticking with a 401(k) alone could really hurt your prospects for financial security in retirement. Instead, consider splitting your money up among several different retirement plans so you can reap the benefits that each provides.

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3 Pitfalls of Only Using a 401(k) for Retirement - The Motley Fool