Start with whether you can join, because the plan can simply be closed to you. The SEC's guidance is explicit that "some companies require that you already own stock in the company or are employed by the company before you may participate in their direct stock plans", and its page on direct investing puts the same point as companies limiting these plans "to employees of the company or existing shareholders". Plans may also set financial thresholds: "some plans require a minimum amount of investment or require you to maintain specific minimums in your account". The SEC twice tells the reader to read the plan's disclosure documents before enrolling, which is where the answers to all of this actually live, since none of it is set by regulation.
The purchase mechanics differ from a brokerage order in three ways that matter. First, you send the money rather than having it swept from a cash balance. The joint SEC and FINRA bulletin on holding securities notes that issuers' transfer agents "typically do not maintain cash accounts for investors in the way that broker-dealers do", so "when you make a purchase, you will need to send the transfer agent the funds to make the purchase"; depending on the plan you may be able to automate that and have the cost deducted from a savings account. Second, you buy by dollar amount: "you may be able to buy stock by investing a specific dollar amount rather than having to pay for an entire share", which means a purchase can leave you holding a fraction of a share, and a fraction bought this way arises inside the company's plan rather than inside the brokerage programs where fractional shares usually come from, so the plan's own documents govern it. Third, the timing is the plan's, and this is the SEC's standing caution about these plans: they "usually will not allow you to buy or sell shares at a specific market price or at a specific time", the company instead buying or selling for the plan at set times, such as daily, weekly or monthly, and at an average market price. The joint bulletin adds the operational reason. Purchases are "typically processed on a 'batch processing' basis, which might mean there is a time lag before the order is executed": an electronically submitted order might be processed "in under 30 minutes", while a paper order's batch cycle "might be longer, possibly extending into the next business day", and "in rapidly changing markets, this could result in receiving a different price than you initially intended."
What the buyer actually holds is the least-discussed part of these plans and the most consequential. The joint bulletin sets out the choices: securities can be held in "street name" book-entry form through an intermediary such as a broker-dealer, or in "registered ownership" form in your own name, and registered ownership "can be represented by a physical certificate or can be in book-entry form at the company (also called the issuer) or its transfer agent (which is often referred to as 'direct registration')". Street-name custody, and precisely what protects it, is covered where it belongs, with brokerage accounts. What registered ownership changes is who you hear from: because you are on the issuer's books as the shareholder, the bulletin says you receive "annual and other reports, dividends, proxy materials, and other entitlements or communications directly from the issuer or its transfer agent", and there is no certificate to lose if the holding is in book-entry form.
Plan shares are not automatically in direct registration, and this is the trap in the arrangement. The bulletin states that purchases of securities you intend to hold in the Direct Registration System "are usually executed under the guidelines of an issuer's stock purchase plan, which uses a broker-dealer to execute the orders", and that "to hold in DRS once the securities are acquired, you would need to instruct the transfer agent to move the securities from the issuer plan to DRS". Two things follow. Sitting in the plan and being registered directly on the issuer's books are different states, and moving from one to the other takes an instruction nobody gives on your behalf. And the plan executes through a broker-dealer of its own, so the familiar description of these plans as buying "without a broker" means without your own brokerage account, not without any broker in the chain.
Getting out has more moving parts than getting in. The SEC notes that "depending on the plan, you may be able to have your shares transferred to your broker to have them sold, but the plan may charge you a fee to do so". From direct registration you "might be able to place sales orders, including market order sales and limit order sales, through the issuer (or its transfer agent), if it has a sales facility that accommodates such types of orders", and the issuer or agent then uses a broker-dealer to execute them. A position can also be moved between direct registration and a brokerage account in either direction: your broker can electronically "pull" it, or the transfer agent can "push" it, the second being used less commonly and possibly requiring the broker's DTC participant number and a medallion signature guarantee from you. The bulletin's standing advice is to ask both the broker and the issuer or transfer agent about the fees before moving anything either way.
On cost, the historic appeal is smaller than it used to be. The SEC's direct investing page says buying this way "saves on commissions, but you may have to pay other fees to the plan, such as fees incurred if you transfer shares to a broker to sell them", and the direct-investment-plans page repeats that "you may have to pay a fee for using the plan's services". The saving that originally motivated these plans was the per-trade commission, and what a retail brokerage charges for an online stock or fund order today is the subject of commission-free trading. So the comparison worth running is the plan's service, sale and transfer fees against what your own brokerage actually charges, rather than against a commission schedule from an earlier era.
Availability is not uniform, and one piece of it is state law rather than federal. The bulletin warns that "not all issuers offer direct registration, some issuers no longer issue physical certificates, and not all brokerage firms accept delivery of physical certificates", and directs the reader to find out from the brokerage firm or the issuer or its transfer agent what is available. On certificates in particular it is precise about the source of the right: "entitlement to a physical securities certificate is a matter of state, not federal, law", so whether you can insist on one depends on the state and on the issuer's own governing documents.