The main disadvantage of a CRM is that it's not usually free. Some providers do offer free plans, but you'll need to upgrade to a paid plan if you want full functionality.
While companies can use spreadsheets or bespoke solutions, a CRM costs a regular subscription fee, which can be expensive for a small business. However, it's often worth the investment to appear more professional.
Alongside that, certain companies may require a more specific service that CRMs don't offer, which is where a bespoke CRM may need to be created if possible. Staff will also need to be trained on using a CRM, which can put extra strain on certain departments.