Boundaryless careers refers to workers in a firm who work across a range of boundaries, be it departmental geographical or inter-company.
It may also refer to a worker whose career goes across a number of companies in a variety of roles such as implementation, consultation and training. It is seen most commonly through careers such as designers and "Gurus".

You can write books on this question, indeed people such as Arthur and Rousseau have, so I'll be brief and focus on the pros and cons.

Individual
Pros -
-Working across departments leads to great personal networks
-Project based approach leads improved management skills
-Working across firms/departments can lead to learning of best practices
Cons
-Not suitable to all people, some prefer more stability
-Job/Industry specific: not suitable for all sectors
-If identity is linked to work then can lead to "boundaryless identity?"
-Requires much "learning on the job" which will not suit all people
-Requires strong social networks in place
-Can be seen as elitist and creating non-boundaryless jobs for other

Firm
Pros
-Can gain talent for specific projects
-Talent can be sent to "fix" problem areas, irrelevant of location/department
-Learning on the job can reduce training costs
Cons
-Difficult for all people in a firm to be boundaryless (a boundaryless coal mine?)
-Can be costly in terms of admin
-Lead to employee challenges if they see others being boundaryless and are unable to be so themselves
-They may lose top talent as they move from firm to firm
-Makes firm employees lose sense of loyalty to firm