When it comes to retirement accounts, the 401(k) has long been the go-to for millions of Americans. Offered through employers, 401(k) plans allow Americans to save and invest for retirement while lowering their taxable income for the year.

A 401(k) is an important tool -- there's no denying that. And the more people use one to put money away for retirement, the better (in most cases). However, before focusing on maxing out your 401(k) contributions, there are downsides you should be aware of.

You may not have the investment flexibility you prefer

One of the biggest downsides to a 401(k) plan is its investment options are usually quite limited. In theory, 401(k) plans are designed to make saving and investing for retirement as simple as possible, which means your plan administrator provides your investment options.

Typically, these include your company's stock (if it's a publicly-traded company), index funds based on market cap, a few bonds, and target-date funds that automatically rebalance to become more conservative as you near retirement. While these options aren't bad by any means, they can be limiting to people who prefer to have a more hands-on approach.

For example, if you wanted to invest in a company you weren't employed by, industry-specific exchange-traded funds (ETFs), or alternative investments, a 401(k) plan likely wouldn't provide those options. This lack of flexibility could stop you from tailoring your retirement portfolio exactly how you see fit and are comfortable with.

With individual retirement accounts (IRAs), however, you can invest in virtually any stock, bond, or ETF like you could in a regular brokerage account.

There isn't much leniency with early withdrawals

You don't want to contribute to a retirement account with intentions of withdrawing money before retirement, but sometimes, life throws a curveball, and that becomes your only choice. A 401(k) is much less forgiving in many of these situations, often charging a 10% early withdrawal penalty (if younger than age 59 1/2) plus any taxes you owe on the withdrawal.

The same 10% fee applies to early withdrawals from an IRA (except Roth IRA contributions), but they offer many more exceptions that make it easier to access your money without penalties.

For instance, first-time homebuyers can withdraw up to $10,000 to put toward their purchase. This can be used for the down payment, closing costs, or other relevant expenses. You can also make penalty-free withdrawals to cover qualified education expenses like tuition, books, and student fees, as well as cover your healthcare premiums while unemployed.

Those exceptions don't apply to a 401(k) plan.

At some point, you have to withdraw from your 401(k)

The required minimum distributions (RMDs) of 401(k) plans can also complicate your retirement planning. The IRS mandates that you begin taking RMDs from your 401(k) at age 73, and the amount is based on your account balance and life expectancy. Not doing so will trigger a 50% penalty on the amount that should have been withdrawn.

RMDs can create several challenges for retirees, especially if you have other income sources that you can live off of (like investments and Social Security) and aren't totally reliant on your 401(k) funds. To begin, RMDs could potentially put you into a higher tax bracket and increase your tax liability.

You could also find yourself in a situation where you have to take RMDs during a market downturn and must liquidate part of your investments at a less-than-ideal time.

A traditional IRA also has RMDs, but you can hold onto the funds in a Roth IRA as long as you'd like and potentially pass them on to a beneficiary.

Being overrated doesn't mean 401(k)s aren't useful

Despite the shortcomings of a 401(k), it's still a useful resource. A good approach is to use a 401(k) as part of a diversified retirement strategy instead of your sole source of savings. You should use other options like IRAs and a brokerage account to complement a 401(k) and cover any areas where it may fall short.

Doing so can allow you to cushion some of its downsides while benefiting from its key advantages: employer matches and tax-deferred growth.