The cr.LK/cr.LP to cr.LP link has a rhythm to it, so it's not too bad. You want to be able to hit it at least most of the time though, because the difference between landing a few lights and an actual combo into Rekka for Fei is huge.
The issue with st.LP is that on many crouching characters (including most shotos), it whiffs if they're in hit or block stun. From a neutral position, it will still connect on the vast majority of the cast, but otherwise, no go. Just know which characters you can use st.LP on and learn to confirm standing with cr.LK and it will make it a lot easier.
The nice thing is that Fei has a very short list of combos to learn. The key ones are crouching Rekka combo, cl.HP after HK/EX Rekku aka Chicken Wing (you can always opt for LP or LK after, but Rekku combos are Fei's highest damaging ones), combo from Tenshin (not overly difficult but still important) and counter-hit cr.MP into LP/EX Rekka. That last combo can be huge. Fishing for a CH cr.MP is one of the most frequent ways I get the Rekka I need to push the opponent into the corner.
The main thing I like about Fei though is that he's a punish monster. Not necessarily in the quality of his punishes, but the quantity. With his 3F cr.LP, 7F LP Rekka, and 10F HP Rekka, he can punish just about everything that's punishable (that doesn't require a super-fast Super/Ultra Combo). Whenever I play other characters there are a lot of cases of me thinking to myself, well Fei can punish that....
Fei Long is overall simple though, so he can get boring after a while.
