Reasons to segregate:
1) The single biggest one is that it firewalls the liability of the businesses from each other. Whether this is important or not for you depends on what the businesses are doing: if it's Regular Internet Stuff then your E&O policy is probably good enough in terms of risk mitigation, but if 1+ of your products are in highly regulated spaces (hello HIPAA, finance, etc) then putting them in their own LLC isn't a crazy solution.
2) If you're religious about doing not just the paper ownership but the business accounts separately for each business, that makes eventually selling or otherwise disposing of them much, much easier. Otherwise you're looking at weeks of work and/or very fun professional services bills when you decide to do the division later.
3) If you have co-founders or investors, or the prospect of getting co-founders or investors, separate legal entities are going to be pretty much required. You don't want them to accidentally get ownership of your side projects; they don't want to own your side projects (ownership is a risk; they know the risks they're signing up for and don't want additional sources of uncontrolled unknown risk).
4) A minor factor, but there is non-zero social friction involved in "We've been talking about my trading name of $FOO but remember that the invoice/contract/etc will be from $BAR, LLC."
Reasons to not segregate:
1) It's a lot of extra work.
2) There's a running cost to keeping an LLC open, both the yearly fees and the operational complexity of maintaining separate books, accounts at various providers, and (if you're doing things in a complicated fashion) keeping up appearances with regards to the LLCs being formally separate from each other.
As an ex-consultant with some accidental knowledge of the payments space: I would be doing double-plus firewalling between any payments startup and anything I'd lose sleep about losing, and I would be happily writing a sizable check right about now to a lawyer rather than taking HN's advice about my compliance obligations and potential sources of risk.
Most small software companies have a single entity and only choose to spin out when a new product becomes a truly independent operational unit, when it receives investment, or (for branding purposes) if it ends up eating the business that spawned it.
Is there any legal ramification to "converting" product X, originally under the umbrella of Y LLC, to its own new X LLC?
Thanks
IANAL or Accountant, but the basic rule I was told was that if there are expenses or revenue in Y LLC for product X then you must take specific steps to legally transfer it to X LLC so that both entities are protected and it is as close to tax neutral as possible. But if you just started creating product X and Y LLC doesn't have any booked revenue or costs associated to it then it is totally different as there is nothing traceable between product X and Y LLC.
So, it depends. In my case for the one where we went through the most hoops, the new LLC "acquired" the Product from the original LLC in a tax neutral way, but contractually (basically an asset purchase agreement). And the Agreement stated the new LLC was taking responsibility for all liabilities past and future in relation to the product etc. It wasn't expensive or hard to do right, just took a little extra help.
But it might be wise to wait until the funding is almost-confirmed before starting with this. The investors may also have specific requirements on where and how the LLC is registered, and you may need to do it over again if you do something else right now.