Even in Free EU Roaming days, I was on Virgin and the offering was absolutely dire. Completely and totally unusable (barely connected to 2G let alone anything better). I just bought a SIM card from a Dutch company. But then had a problem because apparently you needed to have a Dutch ID to activate it so I had to call a Dutch call centre (thankfully they all speak English!) and pretend I lived at a random street in Amsterdam somewhere I googled. But it cost €15 or thereabouts and gave unlimited data for a month (which was more than enough for the trip).
As for on train wifi, my most recent experience was in March trying to work on the train from Wigan to Carlisle. The Avanti WiFi obviously didn't work. It would randomly connect to the log on portal and not get past that. Then it did log on, but just repeatedly disconnected (which was annoying as my laptop then tried to connect thinking it was a good network, when in reality it wasn't). I just deleted the network and used tethering, which also barely worked.
WiFi as a technology has some pretty serious limitations. First is that the original spec 802.11b/g only has 13 "channels" and any other user of the same "channel" as you will interfere with your own signal. Other electronic devices also interfere with the channels. It is preferred in WiFi hotspots though because it is cheap and the signal is strong and travels further than the later spec. But that's not the end of it, because the channels overlap. So in reality you can only achieve 3 interference-free channels on a good day. And that's not the end of it, because some devices are set up to try to use the same channels. And that's not the end of it, because when measuring signal "strength" as opposed to signal "quality", the interference isn't detected. So your device gives you "5 bars" or whatever, finds a channel and thinks that it has a good network and will try to connect to it. But the network could be congested and unusable. And that's not the end of it because even if the network is usable, whether badly or otherwise, it just means your device can connect to the hotspot. It doesn't mean the hotspot itself has a good connection to the internet, and there's no way for your device to know that.
The internet - by design - unlike what are called "circuit switched" networks is a "packet switched" network. So devices just send "packets" of data into the void and fingers crossed they get somewhere and something comes back. A circuit switched network (like original landline phones) is only considered "connected" if the origin and destination are known to have a connection, whereas in packet switching, your origin is *required* to not care whether or not there is anything at the other end.
The more modern 802.11g and later variants of WiFi have channels that don't overlap and many more of them. But the signal is weaker and doesn't travel as far. But - as I said above - that only helps you get a connection from device to hotspot. The hotspot itself has to act as a router and know who is connected and where all their "packets" are to be sent to and from. That's easier said than done and cheaper kit will overload itself if too many people try to connect (e.g. on a train). Because of the packet switching, a device doesn't know if it's properly routed or not, it just sends things out and realises a few seconds later that its data hasn't been acknowledged by anything (because the router dropped the packets or the router itself has no signal). But then all the device does is re-send the data, further overloading the router. And if the router also has to do authentication (the so-called "captive portal") that is not part of the WiFi specifications at all, it is a system crowbarred on usually by intercepting traffic and falsely directing you to the address of the portal for it to log you on, and then trying to remember who you are and know if you are allowed to connect or not. This could be done in a number of ways, but I expect the on-train WiFi just VPN all the traffic to a server somewhere (apparently in Sweden in a lot of cases) which may or may not be fast enough to deal with all the users at the same time. And if it isn't fast enough, your device just gives up and starts again, further overloading the network.
4G and 5G have none of this (they are still packet switched though). The only real issues on those networks is getting a signal in the first place, and too many users at one base station saturating it (by too many, I mean in a festival or at a football match). But obviously a 5G cell tower is an order of magnitude more expensive than even the most expensive WiFi hotspot kit, (tens of thousands just for the ground-based base station kit alone, and then you have the antenna, the construction, power supply, etc. so I'd guess you're well over £100k per station) although it's also an order of magnitude more reliable and can handle thousands of users easily.
), the can easily be a cliff edge condition where so many packets of Wifi data fail to be received, that the system collapses unde the load of retries.
