Pros
Wide variety of projects gives you exposure to many different parts of the construction industry. Building commissioning is basically all Horizon does and they are well regarded in this role. Horizon has the commissioning process well ironed out and you will learn a lot about the building systems that get commissioned. Horizon is commissioning a lot of big and interesting projects.
Cons
Work/life balance is not great and employee turnover is incredibly high. Plan to work 42-50+ hours per week depending upon your role. Expect to be in the office at 8:00 AM every day (regardless of your commute). Night and weekend work is not required, but it happens often, and someone has to do it. Do not plan on working from home since there is a highly inflexible teleworking policy. Paid time off includes your sick days and don't expect to get much PTO. Do not plan on getting much of a bonus, if at all. Plan to turn in a detailed time sheet every week and be ready to answer questions from management if your billable hours are not on target. Promotions happen once a year and you can plan to work in a new role for a while before actually getting the promotion.