Disciple of the Land leves are fine. At low levels it only takes a set or two of them to ding, at later levels it takes more, but let's do the math:

For my level 43 miner, a pair of DoL leves gives about 40-50k exp in rewards. Let's average to 45k.

Now, leves are better than regular gathering for regular exp because each node will have 6 swings, allowing for higher chaining and giving more rewards for any gathering rate boosts you use.

Leves don't let you get HQ items, though. However, they do give a chunk of exp at the end.

So, as to whether or not they are exp efficient we just need to compare of bonus exp from the leves relative to the bonus exp from HQs.

Two leves will take me about 5 mins (or less) to accomplish. In 5 mins, I can usually do, at best, 5 rounds of 4 nodes when gathering regular. Assuming one of those 4 notes gives a bonus gathering attempt, that's 17 gathers per round, so a total of: 85 gathering attempts. Let's assume, charitably, that they're all successful. With an average HQ % of pretty close to 15%, that means one will hit about 13 HQs during that period of time.
Now, at my level I get about 1k exp per HQ hit. So that's 13k exp from HQs if I spent that time doing regular gathering. If I spend that time doing leves, I get more exp from regular gathering and I get a minimum of three times the bonus I would get from HQs from the turn in rewards.

Finally, I don't get to keep the items but I do get gil. Usually about 800 gil per pair of leves. I could probably make more if I farmed the right item (though at some levels there's nothing worth gathering that makes for good experience), but I'm still making decent money. For instance, I have ~50 leves. If I did all of them levelling the rest of the way to 50, I'd make about 25k gil. As far as repair costs go, that's not a bad amount of money. There are some really decent earners at low levels though (shards and crystals) which probably make way more money than leves of that level would. So the results on this front may be more peculiar to the higher levels because of the fact that most items are being undercut like crazy and are near worthless to farm.