Do you have a particular example of that?
Also if it looks expensive with a big saving, then by extension, the fare the TOCs would be charging would have to be mega-expensive!
This might not be quite the scenario envisaged - but could be an example of the "share of savings" feel appearing expensive.
A journey I do quite a bit is London-Inverness. Aside from the sleeper there's one train a day that goes right through - an LNER one and that's the only route that a conventional search will give you an advance fare for. Otherwise you're basically looking at on off-peak. There's no "LNER + connections" type fare for this route, for example.
Anyone who does this semi-regularly is quite likely to understand there's a very obvious split - London to Edinburgh with LNER and then Edinburgh to Inverness with Scotrail. And often you'll find that this will beat the walk-up fare by quite a bit.
So ... if I ask trainsplit to look for tickets, it will quite likely find the split that I already know about, and it might "save" me £50 and charge £5+ for doing that work. But to me, that feels like I'm paying for something I can easily find myself. On the other hand... it might (and sometimes does) find me a more complex split, perhaps a string of different tickets for the LNER leg and/or the scotrail leg. In that case it's a bit different because it's very unlikely I would have found that myself. However... the additional savings in the fare are quite marginal - maybe it beats the "obvious" split by £1.50 (in terms of the raw fares, before adding the fee). But once it adds £5+ back on top of that, it's asking me to pay more than I'd have to pay if I'd just looked up the obvious split options myself.
Another scenario on the same route (I'm just using it as an example because it's one I'm familiar with):
Take the 14th of March - if I look on LNER it'll offer me £79 on the 1200 direct train. For the 1300 train, it'll tell me it's £106.60. Screenshot showing this:
If I ask trainsplit about the same day, it'll offer me as per this screenshot:
£73.42 for the 1200. Well done to trainsplit - they beat LNER. And many people will be quite happy to accept this lower price. Even if they see that £1.19 is taken as a fee - that's not much. I'd pay that rather than faffing about doing it manually.
What if I'd kind of prefer to travel at 1300 though? Trainsplit offers me £81.80, which is more than the straight-up LNER advance on the 1200 service. That's only because of the "share of saving" fee though, which is £4.50, as seen in this screenshot:
If that fee were instead £2, then it would still beat the LNER price.
This is a rather marginal difference perhaps, and you might say a contrived example. But perhaps I could find another day where trainsplit can't beat LNER on the 1200 train price, and then a customer might look at their options and go for the LNER one when trainsplit might have got their custom had the fee been a little lower.
lt’s also worth considering that there are many occasions where the split ticketing engine will be able to produce cheaper tickets that are either not available on Toc websites, or require additional ‘advanced’ options. ln these cases, despite saving you money you won’t have to pay any additional fees because they aren’t split tickets.
In my Inverness example, this sometimes happens when you're offered a routing via the WCML ... a series of advances without any additional fee. I guess because that routing isn't covered by the "any permitted" off-peak?