Maybe Finland is an exception here, but they are definitely tiered by speed, with no 'buckets' of data - Finland is the only country I've personally purchased an actual plan in, I've only used prepaid plans elsewhere in europe.
https://kauppa.saunalahti.fi/#!/matkapuhelinliittymat https://www.dna.fi/liittymat
Now many providers have dropped the "unlimited 2G" data claim and just stating the rate limit after you use your high-speed bucket.