Well they are more popular in the Uk than in US. My problems re 901 are
1 - hard to get manual
batteries and if you do, most 901 PCCs don't seem to accept manuals (and even if you did find manuals, they might still have an open hole in the threaded end)
2 - hole in the side of the atomizer can drip
juice on your fingers.
3 - 901 cartomizers are hard to find in the US (and the ones I saw had a hole in the side). Some KR808D-1 cartos will work but some need an adapter to get airflow
4 - I have had issues with 901 atomizers - hard to clean because the coil compartment is closed off and the best hole is in the side. And the barrel could twist vs the threaded end and cause some plastic armatures inside the coil compartment to block the air hole in the side (the 901's air intake) making the drag bad
In the US if I wanted to use 901 atomizers, I would buy KR808D-1 manual batteries and use the 901 standard resistance atomizer and carts or drip tip, or use KR808D-1 cartomizers on it. KR808D-1 batteries are real 3.7v slim batteries and the modern manuals are fairly well sealed on the threaded end, so they are pretty safe from being damaged of killed by
juice leaks from cartomizers or dripping on an atomizer. A manual battery also lets you control your hit better and makes it easier to get a good hit. What you cannot do is use a scissor grip with 2 fingers to vape.
But if it's your first ecig and you like it you could for for it. Maybe the UK has a better selection of 901 stuff.