Canada

I suspect that, when certain election methods are used, it's possible to make your preferred candidate lose if you express support for them:

instant-runoff voting is also sometimes vulnerable to a paradoxical strategy of ranking a candidate higher to make them lose, due to instant-runoff voting failing the monotonicity criterion.

I can imagine that someone's best choice can be to entirely abstain from voting in some situations. I don't think it's ethical to force people to vote if doing so would harm them.

Making a law about an obligation to vote will probably make future electoral reform harder (since people will have to figure out / get confused about whether a change will make it more likely for them to land in court), and making it hard to change bad systems is surely a bad thing.

Incentivizing someone to show up and just cast a blank ballot could make it harder to detect fraud. For example, it might be convenient to dispose of ballots that someone intended to misuse by mixing them in with the legitimate ballots, and having more blank ballots that are actually legitimate would make it less clear whether something illegal has happened.

"Voting in all federal elections in Australia is a legal obligation for citizens aged 18 and over", but there isn't a very steep penalty for not doing so (and you might even get your name published in a newspaper, which some people might value for its own sake):

Failure to vote in the federal election initially results in a $20 fine as per section 245 of the Act, which can grow if left unpaid to a $220 fine and court fees.

Having early voting and making the "main" voting day be a holiday for a large number of people seems like a good idea, since that makes voting easier for people who want to vote. Hounding people who don't want to vote (regardless of their reasoning) seems like a worse idea.