Vantaggi
Large budgets, large infrastructure projects, looks good on CV, large teams to work with, involves interaction/management of stakeholders such as HEX, GWR, DfT, TFL, etc., decent policies in terms of sickness and leave, large internal job market, multiple offices/depots to choose from.
Svantaggi
All engineering input comes from contractors (NR are just reviewing/validating drawings, authorising works), too many middle-men in a matrix type, highly bureaucratic organisation, very little (if any) operational efficiencies achieved, no decent software support (no ERPs/CRMs/PM tools/etc.), poor strategic HR management, heavy staffing using agency personnel (Bechtel, Turner & Townsend) alongside personal staff, hilarious costs for outsourcing temporary staff compared to perm staff, poor handover techniques in environments with high stuff turnover, literally the opposite model compared to a private-sector medium-sized consultancy in terms of efficiency and staff motivation, I can't think of a project delivered on time and within budget.