Learn English – Which one is more appropriate to use

They are disappointing.

This is present tense and refers to "now".

I've been following the band for three years and they have been disappointing in their last five concerts.

This refers to a period of time in the past that may extend up to the immediate present. If I remember correctly, it is present perfect.

I've been following the band since they started playing and they were disappointing for the first few months.

This is past tense and refers to a point or period in time in the past, but not extending to the present. The event is in the past.

1. American speakers tend to use the present perfect much less than British speakers.
2. Lately I get the feeling that... is just idiomatic AmE. It's not the same as, say, Lately I've been feeling sick ..., which is normal AmE: it's present perfect progressive because it's a consistent feeling. I use the present perfect much more than most AmE speakers, but I wouldn't say Lately I've had the feeling that... unless I didn't have it at the moment of speaking. Lately I've got the feeling... doesn't feel like AmE to me.
3. ... have you noticed how much more time Paul has {spent/been spending}...? is idiomatic AmE, but I suspect that a lot of AmE speakers would say it in simple past: ... did you notice how much more time Paul {spends / has spent / has been spending}...?

The sequence of tenses is difficult for all speakers, native speakers included. What each tense, aspect, mood, voice, etc. is used for is a mystery to most. We just instinctively say what we say and use whatever tense seems to fit. IOW, I don't think it's possible to provide a rule-governed explanation of why people use one tense instead of another.

I agree with Reg Dwight that "there is no rule that 'lately invites the Present Perfect'." I'd say "Lately I feel sad about how vitriolic American politics has become" as soon as I'd say "Lately I've {felt / been feeling} sad about how vitriolic American politics has become". It'd all depend on whether I was feeling sad at the moment or whether I'd been riding a roller coaster of sadness and indifference.