Hardware or OS version, no, it really shouldn't be. App or service provider are the obvious bottlenecks. Both are telling you that they aren't, but being cynical that makes me suspect that one of them is wrong. It is certainly the case that both apps and service providers have limits, and that both vary between apps and providers.
One question: are you just sending a notification to people, or are you having group chats where you want everyone to see everyone else's replies? The reason it matters is that a one-way communication can be done using SMS, whereas if you want everyone to be able to see everyone else's replies you need to use MMS. And while I don't know whether this is the case, I wouldn't be surprised if the limits were different. That's the sort of thing that could also result in misleading answers about what the app or provider's limit is.
It will be much simpler if you are just sending a notification and don't need a multi-way chat, since then as long as you can solve it for you then you're good. Whereas if everyone needs to see everyone's replies solving it for you might not solve it for others. That is where other services (WhatsApp/Signal/Telegram/etc) have an advantage, because they can all handle large groups and will be the same for everyone.
If you really have to use SMS or MMS all I can suggest is trying some other SMS/MMS apps and seeing whether it makes a difference. All SMS/MMS apps use the same system infrastructure, so you can just swap apps very easily. If it's the same for several apps I'd be inclined to suspect it's the carrier.
One thought though: does this fail when you send the message, or does it not allow you to add extra participants? If it's the latter then it may be the app (though if the app is aware of a network limit it might still be the network), whereas if the app allows you to make a group that size but then fails to send that would suggest the carrier (or a very badly-written app!).
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