Regulated financial products impose a specific set of engineering constraints that standard software projects do not. PSD2/PSD3 compliance, strong customer authentication (SCA), and AML/KYC pipelines require specialist knowledge across both technical implementation and legal standards. Security demands are also higher: financial applications are attractive targets for fraud and data breaches, requiring penetration testing, end-to-end encryption, and continuous vulnerability management as baseline practices. Third-party integration complexity is routinely underestimated, with payment gateway APIs, core banking systems, card scheme certification, and credit bureau connections each contributing significant surface area. Regulatory requirements shift over time as well, and architecture decisions made for one directive may need revisiting after the next, which is why long-term engineering partnerships tend to outperform one-time project engagements.