These two special episodes of the second season of Yama no Susume are pretty different from each other and have to be treated separately.
-- Bra no Susume --
The first one is similar in concept to the special episode of the first season, in the fact that it's basically an episode like the other ones but with added fanservice (which is extremely mild in the actual anime): this is something that can be seen everywhere in the universe of moe shows, and even if it's rather surprising to see fanservice in Yama no Susume which is praiseworthy for its lack of boob-grabbing and similar tropes it's
not something that would bother the average viewer too much; Bra no Susume has a complete plot which departs from mountaineering and focuses only on cuteness (bra cuteness) and, while it would probably be a little bit clashing if inserted in the middle of the regular anime, as a special it's nothing out of the ordinary.
In the end, if you like seeing the characters from the actual show and you don't mind fanservice then this episode will be pretty enjoyable.
-- Yama no Susume Best Ten --
The second one departs again from the style of the actual anime, but in a different way from Bra no Susume: Best Ten is designed as a behind the scenes where the characters from Yama no Susume comment on the show itself, and this special is filled with breakings of the fourth wall and over-the-top sequences that alter the telling of some key moments of the anime. I don't intrinsecally despise these solutions when they are well executed and there's plenty of slice of life shows in which they would work, but I didn't feel like Yama no Susume was one of them: while I could accept the emergence of fanservice in its specials, these wacky sequences contrast very much with the generally relaxed feeling of the rest of the anime, in a way that seemed forced to me; it's a kind of humour that gives a very different feeling from the normal episodes and I don't think it suits well with the whole spirit of the show.
If I had the possibility I would still tell my past self to watch it for the sake of completion, but I would warn myself against its general weakness, silently wishing for another kind of ending to this very good season.