I guess they are all popular and pretty similar so a competent bookkeeper should be familiar with it
Never make assumptions. What online software are you talking about? A LOT of book-keepers have very little experience of the relatively recent cloud innovations like Xero, Quickbooks, etc. Lots of book-keepers use dinosaur software like desktop Sage or even spreadsheets.
I have one client using Xero who's gone through no fewer than 5 "reputable/experienced" book-keepers who just hadn't a clue. Those 5 weren't from shop window or facebook adverts, they were so-called professional book-keepers, some were members of the AAT and ICB, with their own websites, etc. When you scratch the surface, they talked the talk about online/cloud software but didn't actually use it that much, if at all. The client in question was tech-savvy and used Xero because of the integration with other online systems, such as his bank, online retail platforms, credit card processing, etc. None of the 5 book-keepers could handle any of that - they wanted the client to "teach" them how it all worked. The client in question has now decided to just carry on doing it himself.
The thing with book-keepers is that Cloud/Online subscriptions are expensive, cloud/online data entry usually takes longer. So, book-keepers are more likely (historically) to take your paperwork and process it on their own system. Most have "multi-company" desktop software where they don't pay any more for more companies. Some of the desktop software is incredibly quick/easy to use. In my practice, we use VT for book-keeping. It's a single annual fee regardless of number of firms. It's quick and easy for data entry, incredibly quick/easy to correct, etc. That's why book-keepers tend not to use online/cloud systems which are usually slower and more costly and why they often have little experience/ability with the cloud/online systems that a business owner would use.
So as I said at the beginning, make no assumptions. Check thoroughly that they know all the ins and outs of whatever cloud/online system that you're using. Last thing you want is them making loads of errors that are going to mean your accountant takes longer at the year end to prepare the accounts/returns meaning far higher accountancy fees to sort out the mistakes!