Well I think it's different issues for every game mode.
First of all though: The menus.
While Warband's menus loon dated by today's standard, I think there were two advantages:
The multiplayer button was in the same menu as the campaign button. In Warband after hours of singleplayer at some point this button was making me curious. I don't think that happens mich in Bannerlord.
On that note- what's the purpose of the friendslist in the Multiplayer. I can see that my friends are in Singleplayer but I have no way of contacting or even inviting them to my game. Actually in Singleplayer I can't even see my friends that are in MP. So why is that a thing in the first place? It feels like a missed opportunity.
The other issue is the multiplayer menu.
In Warband you were greeted with the server list. You could see where the most action was or look for the game mode most suitable. In Bannerlord that's hidden.
Sure it's only one click away but that might be already to many.
The game modes. So I'll start with best to worst:
Skirmish: Skirmish is imho the most balanced and most thought out game mode we have. I think classes worl well there and nowadays every class is getting used reguarly.
It's problems are that it's not beginner or solo queue friendly.
That can only be solved when the playerbase for that game mode grows. Which doesn't happen as it's not beginner friendly
Team-Deathmatch:
I think TDM is okay - even with the class system. But that comes my personal view. When I join TDM I expect an chaotic, mindless slaugther filled with 2-hander spammers and cav. If that's what you expect from the game TDM works.
If you want a strategic game mode that you play with your friends or clan- it's not TDM.
The factions aren't well balanced and nothing and everything you do feels meaningless.
Contrary to most other people I don't think the class system is a big problem for the game mode. -Well, again, if you look for a mindless slaugther. If players don't gain everything from playing defensivly they obviously go for the loadout with most damage output - so cav or two handers.
I think TDM is probably the worst first step into the multiplayer. I think it encourages bad habits and is a rather frustrating experience.
Duel:
Duel isn't a real game mode yet but I want to mention it here.
Duel works, barely but it works.
The gameplay is quick and fun but people are expected to know the server rules. The class system is again no issue since most players use the same classes anyway.
Duel is probably the best place to improve your melee skills but at the same time it's not a good entry point.
But thanks to it's exlusivity you don't see many new players here.
Siege:
Siege could and should be the go to game mode for beginners. It's atmospheric, it's about teamplay and it favors players survivabilty.
Which is super frustrating because imho it's ruined by the class system. In Siege players should have a reason to try to get better armor and shields. But heavy infantry costs too much gold. So most people that aren't archers go for light infantry (which is garbage) or shock infantry. And once everybody uses shock infantry, the few heavy inf players just get overwhelmed.
Here the class system makes players pick an offensive class that is not actually synergizing with the game mode.
In Warband that was less of an issue.
Players were able to get relativly bulky from the very beginning which made it harder for offensive builts from the get go.
Additionaly, playing together with friends and clans is not as easy as it should be. Players from my group don't get highlighted and players still can't pick their spawnpoints.
So the game mode that is all about staying alive, cooperation and teamplay doesn't let you have any of these things.
Additionaly the factions aren't well balanced either.
In my eyes Siege is hardly better than TDM for new / solo players.
By the way I wonder how TW wants to use voicechat in those massive servers.
Lastly Captain:
On first glance the ideal bringe between singleplayer and multiplayer.
But for a gamemode that's all about strategy and making use of the correct units for the correct situation the balance is really horrible.
Additionaly in a game mode that's all about strategy most units are useless.
Captain is from a gameplay perspective in a very bad spot.
That, together with the importance of teamplay and communication, makes it another bad entry for beginners and solo players.
To sum it up: There is not a single Mode that is beginnerfriendly. Either you're matched against way better players or you are thrown into a frustrating chaotic experience.
The best way to lern multiplayer (and I am serious here) is the training camp at the beginning of the SP campaign.
But even training there can't really prepare you.