When you're a small firm you have little choice but to have soup-to-nuts administrators. As you grow IMO it really makes more sense to consolidate certain services that are peripheral to the ongoing admin cycle. Functions such as trust accounting, loan and distribution processing, onboarding and plan documents are all tasks that lend themselves to being segregated. We evolved this way over 25 years.
It is really difficult to find administrators who are experts at everything, and the really good ones are expensive. Training them to do documents, assemble pdf packages, deal with mundane tasks like withholding is not a good use of their time and about the most expensive way a firm can handle these functions.
Creating separate functions creates a career path for new hires who find they like the business and want to learn more. As you get bigger it is more and more difficult to find well trained experienced administrators.
Separating functions and creating processes improves the client experience and makes it more predictable regardless of who is handling the plan. I know firms where some administrators are still using the Datair DOS system and others are using the Win system. When a firm has multiple clients with an advisor or CPA this creates confusion when they see vastly different reports.
Quality control. With separate functions the people in the separate departments can be really good at handling TA, loans, distributions or documents. This makes them faster and more accurate. Also a firm with separate silos very vulnerable when an administrator leaves, is on vacation, etc.
Onboarding should absolutely be separated from regular admin in any firm that really wants to grow. Having a predictable, repeatable and scalable new business process is essential to create trust with referral sources and keep business development people focused on opportunities rather than getting involved in the minutiae of a takeover. And administrators with caseloads are on a treadmill with deadlines 2x per month and their own clients to whom they've made their own promises and commitments. Dropping new business on them disrupts them and they often get shuffled to the bottom of the pile. Renee Connor at PensionPro published a very good article on this a couple months ago.
Just my opinion.