I'm sceptical about these reports that the missiles/drones came from Iranian territory...it's probably just a media smokescreen.
For all we know these missiles and drones could have been launched from inside Saudi territory... I mean the Houthis are claiming they did it and they share a border with Saudi Arabia... it is not their first attack on targets inside Saudi Arabia that were not intercepted and therefore likely not detected either.
If they came from Iran it is actually much worse than if the houthis launched the attack from Yemen because on paper Iran is a rather more serious threat to SA than Yemen is and if they didn't see it coming that suggests some really serious gaps in their air defences... which they have literally spent billions on...
As for a hypothetical conflict in Europe, we can only speculate. Most European countries don't really have large Air-Defence forces. But I guess there would be more things like AWACS and other types of Recon planes flying around
Probably the best western IADS are on their ships and Saudi Arabia has a large stretch of flat open water between them and Iran where you really can't hide a missile or drone... europe doesn't even have that advantage...
Saudi Arabia is western oriented when it comes to air defence... it is air attack with the air component doing double duty as air defence but most of the time... NOT.
Most of their air power will be being used for attacks and wont be available to defend anything but themselves... they need to buy more planes... 200 F-35s would end the regime just on cost of operations.
S-400 is better. It can use the same missiles but it also has more radars and better ones of all sort when deployed.
S-350 seems to be much simplier and with only 1 engagement radar and 1 surveillance radar. But I'm not sure about that. To be verified.
But the point is that both S-400 and S-350 batteries can have S-350 missiles... and being able to have four times more S-350s than S-400s means if they do have S-350s they can be equipped with rather more missiles.
For low flying targets a battery on its own likely wont be detecting drones and cruise missiles at much more than 20-30km at best... but with an AWACS platform and other radars it could engage low flying targets out to the max missile range of 150km or so... which is plenty if the battery is located near the targets.
With air power supporting a fighter like an Su-35 with 14 weapon hard points... assuming 2 pylons are taken up with ESM pods then it can carry 12 air to air missiles to take down drones and cruise missiles.
Some sort of air launched Pantsir-SM missile... the small quad missiles in each tube would be ideal for fighter aircraft to take on cruise missiles and drones... perhaps a cluster of four tubes per main wing pylon... say six pylons with four missile tubes on each pylon and four missiles in each tube would equate to 24 tubes with four slim missiles in each tube... so guided 96 missiles... with an air launch they would probably be 25-30km range command guided missiles... which would be pretty useful to deal with a swarm attack.
Being non stealthy, carrying these tubes wont ruin their performance... they would probably be rather light weight... a light aircraft like a Yak-130 could carry two tubes per pylon on four wing pylons, for 8 tubes with 32 missiles per aircraft and space for a centreline targeting pod and wing tip R-74s... or perhaps twin pack Igla-S or Verba missiles.
The thing is that you can't afford to have these aircraft flying around all the time in case there is an attack... it is called a surprise attack for a reason... and if you leave any gaps then you are just asking to be surprised by an attack from that direction... you could put cruise missiles and drones on a cargo ship and launch from any direction you like...
I would suspect the S-350 comes with some pretty sophisticated radars and air defence sensors... likely including IIR sensors because the 9M100 will be IIR guided.
BUK is used by the Russian Army (and Navy) so it would be defending operational units but not used for general air defence of the country.